Progress in Nucleic Acid Research and Molecular Biology 43:135-204, 1992

I. New Technology and Old Theories in the Search for the Causes of Disease

   A. A New Generation of Virologists Presents Latent Viruses as Pathogens

   B. From Retroviral to Cellular Oncogenes-The Oncogene Hypothesis

   C. From Autonomous Pathogens to Multifactorial Causes of Disease

   D. The Search for Alternative Hypotheses

II. Inactive Viruses and Diseases Resulting from the Loss of Cells

   A. Human Immunodeficiency Virus (HIV) and AIDS

       1. The Virus-AIDS Hypothesis

       2. The Drug-AIDS Hypothesis

       3. The Drug- versus the Virus-AIDS Hypothesis

   B. Hepatitis C Virus and Non-A Non-B Hepatitis

   C. Measles Virus, HIV, and Subacute Scleroting Panencephalitis

   D. Phantom Viruses and Neurological Disease

III. Viruses as Causes of Clonal Cancer

   A. Human T-cell Leukemia Virus and Adult T-cell Leukemia

   B. Herpes Virus, Papilloma Viruses, and Cervical Cancer

   C. Hepatitis B Virus and Liver Carcinoma

   D. Epstein-Barr Virus and Burkitt's Lymphoma

IV. Mutated Oncogenes, Anti-oncogenes, and Cancer

   A. Mutated Proto-myc Genes and Burkitt's Lymphoma

   B. Rearranged Proto-abl Genes and Myelogenous Leukemia

   C. Point-mutated Proto-ras Genes and Cancer

       1. The Original ras-Cancer Hypothesis Postulates a First order Mechanism of
           Transformation

       2. Ad hoc ras-Cancer Hypotheses Postulating Second- and Higher-order Mechanisms
          of Transformation

   D. int Genes with Integrated Mouse Retroviruses and Mouse Mammary Carcinomas

   E. Constitutive Oncogenes, Mutated Anti-oncogenes, and Cancer

V. Conclusions

   A. Evidence That Latent Viruses and Mutated Cellular Genes Are Pathogenic Is Circumstantial

   B. Helper Genes and Cofactors to Close the Activity, Infectivity, and Specificity Gaps of
        Hypothetical Pathogens

VI. Alternative Hypotheses

   A. Latent Viruses as Harmless Passengers

   B. Drugs as Alternatives to Hypothetical Viral Pathogens

   C. Mutated Genes and Latent Viruses as Trivial Genetic Scars of Cancer Cells

   D. Cancer by Somatic Gene Mutations Unconfirmed

   E. Chromosome Abnormalities as Causes of Cancer

References

"Circumstantial evidence is a very tricky thing," answered Holmes, thoughtfully. "It may seem to point very straight to one thing, but if you shift your point of view a little, you may find it pointing in an equally uncompromising manner to something entirely different.... There is nothing more deceptive than an obvious fact...."

-Sir Arthur Conan Doyle, in The Boscombe Valley Mystery, 1928

The scientific community has been virtually unanimous in admiring its recent triumphs in biotechnology-above all, the detection and amplification of minute amounts of materials into workable and marketable products. However, in clinical diagnostic applications, the new detection methods have become a mixed blessing, which benefits medical scientists but not necessarily their clients. Since rare signals have become just as detectable as abundant ones, many latent viruses have been detected and have been assumed to be just as pathogenic as active prototypes (1-3). Likewise, cellular mutations have become detectable that do not, or just barely, affect the function and activity of genes. Yet when the affected genes are structurally related to retroviral oncogenes, they are assumed to be just as oncogenic as highly active retroviral oncogenes (1, 4-8). However, the evidence for these hypotheses is only circumstantial-based on structural similarities to classical pathogenic viruses and viral oncogenes. Thus, without direct proof, these hypotheses may open the doors to psychologically harmful prognoses and clinically harmful prevention programs, termed "molecular genetics at the bedside" by Bishop (9).

I. New Technology and Old Theories
in the Search for the Causes of Disease

A. A New Generation of Virologists
Presents Latent Viruses as Pathogens

Although viral epidemics have all but disappeared in the Western world since polio was eliminated with vaccines in the 1950s, the number of viruses currently discovered and studied by virologists has reached epidemic proportions. For example, zealous virus hunters have been able to detect by ultrasensitive biological and biotechnical methods latent viruses that are neutralized by antiviral immunity in diseases such as AIDS, leukemias, lymphomas, hepatomas, hepatitis, cervical cancers, encephalitis, and many others (1,3). Their proposals that latent viruses cause these diseases are widely accepted, because from the days when only the most pathogenic and abundant viruses were detectable, all viruses still have the reputation of being pathogens.

However, the diseases with which these newly discovered latent viruses are associated are not contagious-unless one makes bizarre assumptions. One assumption postulates that these viruses are "slow viruses" or "lentiviruses" causing diseases only up to 55 years after infection and only after they are neutralized by antibodies (see Sections II and III). Yet all of these viruses replicate and are immunogenic within weeks, not years, after infection just like conventional viruses. Another assumption is that these viruses can shift from a nonpathogenic dormant state to a pathogenic state without increasing their biochemical activity or abundance.

A case in point is the assumption that AIDS is caused by a virus. There were over 160,000 AIDS patients in the U.S. in the last 10 years, and there is no antiviral vaccine or drug. Yet at the time of this writing there is not even one confirmed case of a health care worker who contracted AIDS from a patient, nor of a scientist who contracted AIDS from the "AIDS virus" that is propagated in hundreds of research laboratories! The AIDS virus is just as inactive in patients as it is in asymptomatic virus carriers (see Section II).

Such assumptions are not compatible with classical criteria of viral pathogenicity. Conventional viruses are very active, abundant and replicating in many cells that are killed or transformed when they cause diseases such as polio, flu, measles, mumps, hepatitis, herpes, Rous sarcoma, and many others (3, 10-12). Likewise, SV40 and adenoviruses inundate many cells with viral T-antigens when they cause tumors, even though the respective host animals are not permissive for viral replication (13). Pathogenicity by these classical viruses results from high biochemical activity in large numbers of cells. These viruses are not pathogenic when they are latent or infect only small numbers of cells. Indeed, even the most pathogenic viruses depend for their survival on asymptomatic infections in which they are highly active in small numbers of cells before they are stopped by antiviral immunity, the reason that such infections are asymptomatic (3).

Furthermore, all conventional viruses are maximally pathogenic within weeks or months after infection before they are neutralized by antiviral immunity, causing disease as soon as they reach pathogenic thresholds in the host (10-12). In rare cases, they may be reactivated to resume replication, and hence pathogenicity, long after they are neutralized by antiviral immunity (e.g., the herpes simplex virus). Reactivation typically follows a transient immunodeficiency acquired by another primary disease or other immunosuppressive conditions (12). Except for these instances of viral reactivation, there are no known examples of viruses that cause diseases only after a long latent period and only after they have been neutralized by antibodies.

Thus, the evidence that latent viruses can be pathogenic is only circumstantial, based on structural similarities between latent viruses and active, pathogenic viral prototypes. Further, these hypotheses are based on the epidemiological evidence that latent viruses occur, or appear to occur, in diseases at a higher rate than would be expected from random infection (3, 14, 15) (see Section V).

B. From Retroviral to Cellular Oncogenes-
The Oncogene Hypothesis

New technology detecting point-mutations, deletions, and truncations of cellular genes and latent or defective viruses put new life in the somatic mutation hypothesis of cancer (16). It was postulated in 1969 by Huebner and Todaro that latent viruses and covert cancer genes preexist in normal cells and are "activated" to cancer genes and cancer viruses by mutation (17). The proposal became known as the oncogene hypothesis. The discoveries in 1970 of retroviral oncogenes (18, 19) and in 1973 of cellular genes from which the coding regions of retroviral oncogenes are derived (20-22) put the oncogene hypothesis to its first test. It was proposed that mutation turns those genes from which the coding regions of retroviral oncogenes are derived into equivalents of viral oncogenes (6). These genes are now called either proto-onc genes or cellular oncogenes (1, 5-8, 23, 24) or even "enemies within" the cell (25). And mutated cellular oncogenes are euphemistically termed "activated" cellular oncogenes (1, 5-8).

Examples of "activated" oncogenes are point-mutated proto-ras genes that are thought to be bladder or colon cancer genes (23, 26-28), truncated proto-myc genes that are thought to be Burkitt's lymphoma genes (29, 30), proto-myc genes with retroviruses integrated upstream (31) and downstream (32) that are thought to be avian lymphoma genes, and rearranged proto-abl genes that are thought to be myelogenous leukemia genes (7, 8, 33). By analogy to proto-onc genes, even genes that are not related to retroviral onc genes are now thought to be "activated" oncogenes if mutated by provirus integration, like the int genes of mouse mammary tumors with retroviruses integrated within or nearby (5, 8, 34).

However, mutated proto-onc genes and int genes with integrated retroviruses are either just as active or only slightly more active than their normal counterparts (see Section IV). Moreover, the mutant genes from tumors do not transform cells upon transfection. By contrast, proviral DNA copies of retroviral oncogenes transform susceptible cells and are about 100 times more active than normal proto-onc genes (24, 35-38). During the last 5 years, the transforming function of retroviral oncogenes, including those of Rous sarcoma, Harvey sarcoma, and MC29 and MH2 carcinoma viruses, has been shown to depend absolutely on transcriptional activity, rather than on mutations in the coding region (39-44). This high transcriptional activity of retroviral oncogenes results from retroviral promoters.

The latest modification of the oncogene hypothesis, the antioncogene hypothesis, proposes that constitutively active, but as yet unnamed, oncogenes are "activated" by mutational inactivation of tumor suppressors or anti-oncogenes (8, 9, 45). Examples are the retinoblastoma and p53 anti-oncogenes that are thought to cause retinoblastoma (45) and colon cancer (46) if they are inactivated by point-mutation, truncation, or deletion. However, unmutated antioncogenes do not revert tumor cells to normal (see Section IV).

Thus, the evidence for these hypotheses is only circumstantial, based on structural similarities between mutated "cellular oncogenes" displaying a normal level of activity and about 100 times more active viral oncogenes. Further, these hypotheses are based on the epidemiological evidence that mutated genes occur, or appear to occur, in diseases at a much higher rate than would be expected from spontaneous mutation (4, 5, 7, 28, 47) (see Section V).

C. From Autonomous Pathogens
to Multifactorial Causes of Disease

In view of the apparent non-equivalence between the postulated pathogens and their prototypes, the original hypotheses have been supplemented by ad hoc hypotheses. Typically, these ad hoc hypotheses postulate second- or even higher-order mechanisms of pathogenesis that include cofactors and helper genes, in contrast to the classical prototypes, which all follow first-order mechanisms of pathogenesis. Moreover, the putative helper genes, like the putative primary pathogens, are not disease-specific, because they are also found in asymptomatic subjects. Indeed, "cofactors" are euphemisms for new hypotheses, which grant face-saving roles to failing incumbents with large constituencies.

D. The Search for Alternative Hypotheses

In the following, we have reinvestigated the evidence for the claims that latent viruses and mutated genes are pathogenic. Since the available evidence for pathogenicity is insufficient, we conclude that the latent viruses and mutated genes must be considered innocent until proven guilty.

Since falsification creates a vacuum, we have attempted to present brief alternatives, drawing in most cases from published work. However, in the case of AIDS, we have documented an alternative to the virus-AIDS hypothesis more extensively, because there is hardly any mention of alternatives in the over 60,000 papers published on the AIDS virus and AIDS since 1983 (48). By challenging currently unproductive hypotheses and by providing falsifiable alternatives, we hope to contribute to the search for what really causes these diseases.

II. Inactive Viruses and Diseases
Resulting from the Loss of Cells

A. Human Immunodeficiency Virus (HIV) and AIDS

AIDS is a new syndrome of 25 previously known diseases (49-52). In America, 61% are microbial diseases such as pneumonia, candidiasis, tuberculosis, cytomegalovirus, and herpes virus disease (50, 52) that result from immunodeficiency due to a severe depletion of T-cells (49, 51). The remaining 39% of AIDS diseases are dementia, wasting disease, Kaposi sarcoma, and lymphoma, which are not consistently associated with immunodeficiency and microbes (52-54). In the U.S., 32% of AIDS patients are intravenous drug users (52, 55), about 60% are male homosexuals (52) who frequently used drugs as aphrodisiacs (54, 56-64, 103), and most of the remainder have severe clinical or congenital deficiencies, including hemophilia (52, 54, 61). Over 80% of the American AIDS patients are 20- to 44-year olds, of which about 90% are males (52). Different AIDS-risk groups have different AIDS diseases. For example, homosexuals have 20 times more Kaposi sarcoma than other AIDS patients (65), intravenous drug users have a proclivity for tuberculosis (66, 67), "crack" (cocaine) smokers for pneumonia (68), and users of the cytotoxic DNA-chain-terminator AZT, prescribed to inhibit HIV, for anemia, nausea, and lymphoma (69-71).

About 50% of all American AIDS patients are currently confirmed to have antibodies to a retrovirus, termed human immunodefieieney virus (HIV) (51, 54, 72). However, all AIDS diseases occur in all risk groups in the absence of HIV (see Section II,A,3) (54). In the U.S., HIV is fixed to an extremely constant reservoir of about 1 million carriers, ever since 1985, when it became possible to detect antibody against HIV (the "AIDS test") (54, 73). HIV is naturally transmitted from mother to child, like other retroviruses, at an efficiency of about 50% (54). This efficiency might be higher than serological tests indicate, because some proviruses of other perinatally transmitted human retroviruses only become immunogenic with advanced age (54) (see Section III). Sex is another natural mode of transmission. However, it is highly inefficient, depending on an average of about 1000 sexual contacts (54, 74), because there is no HIV provirus detectable, even with the polymerase chain reaction (PCR), in semen in 24 out of 25 HIVpositive men (75). Since 1987, when AIDS was given its current definition (50), about 30,000, or 3% of the 1 million Americans infected by HIV (53, 54, 73), develop AIDS annually (52).

1. The Virus-AIDS Hypothesis

Currently, most medical scientists believe that AIDS is caused by HIV (51). The hypothesis assumes: (i) that AIDS is new because HIV is thought to be new in all countries with AIDS (14, 51); (ii) that AIDS is acquired by sexual and parenteral transmission of HIV; (iii) that HIV causes immunodeficiency by killing infected T-cells; (iv) that 50-100% of HIV infections lead to fatal AIDS diseases; (v) that AIDS occurs on average only 10 years after antibodies to HIV appear (a positive "AIDS test"), to reconcile the low (3%) morbidity with the large number of asymptomatic HIV carriers; (vi) that antibodies to HIV do not neutralize the virus (53, 76, 77), to reconcile AIDS with antibodies to HIV; and (vii) that all unrelated AIDS diseases are caused by the same HIV (49, 51, 54, 78).

In view of this hypothesis, AIDS has been defined exclusively by the association of the 25 indicator diseases with antibody to HIV (50, 51, 54). Further, "safe sex" (49, 51) and "clean injection equipment" for recreational drugs (55) are recommended as AIDS prophylaxis for uninfected persons, and the cytotoxic DNA-chain-terminator, 3'azidothymidine (AZT) is prescribed to infected healthy, as well as sick, persons to inhibit HIV (51, 71, 80a, 79, 80). The presence of antibody to HIV in a healthy person is interpreted as a prognosis for AIDS. Testing and counseling are provided routinely to applicants of the U.S. Job Corps (81). Several countries, including the U.S. and China, bar entry to HIV-positive persons. And a negative "AIDS test" for antibodies to HIV has become mandatory in the U.S. since 1985 for the approximately 12 million blood donations that are collected annually (82) by the American blood banks and the Red Cross (Irwin Memorial Blood Bank, San Francisco, personal communication, 1990) and for admission to the U.S. Army (73, 83).

Each of the seven assumptions of the virus-AIDS hypothesis can be challenged on epidemiological and virological grounds:

1. Since all new microbes spread exponentially in a population (11), the complete failure of HIV to spread from its 1985 level, when it became first detectable, indicates that the American "HIV epidemic" is old. This is particularly compelling if one considers that there is no antiviral vaccine and no antiviral drug. Thus, HIV is not new in the U.S.

2. Given that procreative sex is about 10% efficient (3 days per month) and sexual transmission of HIV only 0.1%, it follows that HIV depends on perinatal transmission for its survival (54). If HIV survives naturally via perinatal transmission, it cannot be pathogenic by itself, just like all other perinatally transmitted parasites (12)-except if one assumes latent periods that exceed the normal generation time of humans. Indeed, chimpanzees experimentally inoculated and health care workers accidentally inoculated with HIV do not develop AIDS (51, 54). Thus, sexual transmission of HIV cannot be a sufficient cause for AIDS.

3. Since no more than 1 in 500 T-cells of AIDS patients ever contains a DNA provirus of HIV and over 99% of infected T-cells survive infection (84), and since about 1 in 25 T-cells is regenerated during the 2 days it takes a retrovirus to infect a cell, HIV infection cannot be responsible for the loss of T-cells in AIDS (53). Thus, HIV, like all other retroviruses, does not kill cells (53, 85, 86). Indeed, HIV is propagated commercially for the "AIDS test" in cultured lines of the same human T-cells that it is said to kill in vivo (87).

4. The assumption that HIV is 50-100% fatal within 10 years cannot be correct, because about 1 million Americans carry HIV since 1985 but only about 30,000 develop AIDS annually since 1987, when AIDS received its current definition (50). Instead, it would take 33 years for all U.S. HIV carriers to develop AIDS diseases based on the current data (3% per year). An average latent period of 10 years would predict that 100,000 Americans would develop AIDS in 1 year.

5. Since viruses, as self-replicating toxins, are all fast immunogens and thus potentially fast pathogens, but AIDS diseases are estimated to occur on average only 10 years after HIV is neutralized by antiviral antibodies, the assumption that HIV needs 10 years to cause AIDS is arbitrary. The long intervals between infection and AIDS probably indicate that HIV is not even necessary for AIDS, because there is no "late" HIV activity, and because antibodies continue to neutralize the virus during AIDS (53, 54).

6. The complete absence of free HIV in nearly all AIDS patients (53, 54, 88)-the reason that the isolation of HIV had escalated into an international scandal (89, 90)-invalidates the assumption that antibodies to HIV do not neutralize HIV. Indeed, antiviral immunity effectively restricts HIV in AIDS patients (91, 92) to 1 provirus in about 500 T-cells, and viral activity to less than 1 in 10,000 T-cells (53, 54, 84).

7. Since all AIDS diseases occur in the absence of HIV in intravenous drug users, homosexuals, and hemophiliacs, HIV is not even necessary for AIDS diseases-except for their classification as AIDS (53, 54).

Because of the many virological and epidemiological inconsistencies of the virus-AIDS hypothesis, some, notably Montagnier (93) and recently Maddox (94-96), have proposed that HIV is not sufficient for AIDS. Accordingly, a number of "cofactors" such as mycoplasmas (85, 93) and other viruses (15, 76) have been postulated as helping HIV to cause AIDS. However, there is no consensus at this time about a specific cofactor that would be sufficient to cause AIDS in combination with HIV (76, 93). Moreover, there is not even one plausible hypothesis as to how a latent retrovirus such as HIV, which is present in no more than 1 in 500 T-cells, could possibly help another microbe to cause AIDS that, by itself, is not able to do so.

Indeed, there are at least six inconsistencies between AIDS and infectious disease:

1. Paradoxically, there is not even one case reported in the scientific literature of a health care worker who contracted AIDS from a patient, although there were over 200,000 AIDS patients in the U.S. in the last 10 years (52). Likewise, not even one scientist contracted AIDS from the "AIDS virus" or from other microbes from AIDS patients, which are propagated in hundreds of research laboratories and companies (53, 54, 87).

2. All new infectious diseases spread exponentially in susceptible populations (11). However, despite widespread alarm, AIDS claims since 1987 only about 30,000 or 0.03% per year from a reservoir of over 100 million susceptible, sexually active Americans. This is particularly paradoxical for a presumably infectious syndrome, because conventional venereal diseases are increasing in the U.S. (97) and because there is no anti-HIV vaccine and no anti-HIV drug.

3. The distribution of all infectious venereal diseases is almost even between the sexes (98). By contrast, 90% of American AIDS is restricted to males since 1981 (52). This is incompatible with infectious venereal disease.

4. Almost all (94%) of the Americans who develop AIDS have been subject to abnormal health risks (52). These risks include either long-term consumption of recreational, psychoactive, and aphrodisiac drugs and anti-HIV drugs such as the cytocidal DNA chain-terminator AZT (see below) or congenital or acquired deficiencies such as hemophilia (52, 54). This indicates that specific health risks are necessary for AIDS.

5. The observations that distinct AIDS-risk groups have distinct AIDS diseases-e.g., homosexuals having 20 times more Kaposi sarcoma than HIV carriers from other risk groups (65), intravenous drug users having a proclivity for tuberculosis (66, 67), "crack" (cocaine) smokers for pneumonia (68), and AZT users for anemia, nausea, and lymphoma (69-71)-are also difficult to reconcile with a single infectious cause.

6. All AIDS diseases occur in all AIDS-risk groups in the absence of HIV (54).

In view of these inconsistencies between AIDS and infectious disease and the total lack of a common active microbe in AIDS, several investigators, including us, have concluded that AIDS may not be infectious (54, 56-62, 99-102).

2. The Drug-AIDS Hypothesis

An alternative hypothesis proposes that American AIDS diseases, above their normal background, are the result of the long-term consumption of (a) intravenous and (b) oral recreational drugs, and (c) anti-HIV drugs (54, 60, 103). The following epidemiological and drug-toxicity data support this hypothesis.

a. Intravenous Recreational Drugs. Currently, 32% of the American AIDS patients come from groups that use intravenous drugs such as heroin, cocaine, and others (52, 55). This group includes about 75% of the heterosexual AIDS cases, 71% of the females with AIDS, and over 10% of the male homosexuals and hemophiliacs with AIDS (52, 55). In addition, about 50% of American children with AIDS were born to mothers who are confirmed intravenous drug users and another 20% to mothers who had "sex with intravenous drug users" and are thus likely users themselves (52, 55). Likewise, 33% of European AIDS patients are intravenous drug users (104).

b. Oral Recreational Drugs. Approximately 60% of the American AIDS patients are 20- to 44-year-old male homosexuals (52). The following evidence indicates that they come from groups who use oral psychoactive and aphrodisiac drugs. A survey of 3916 self-identified American homosexual men, the largest of its kind, reported in 1990 that 83% had used one, and about 60% two or more, drugs with sex during the previous 6 months (105). These drugs include nitrite and ethylchloride inhalants, cocaine, amphetamines, methaqualone, lysergic acid, phenylcyclidine, and more (59, 61-63, 101,105-112). A study of 359 homosexual men from San Francisco reported in 1987 that 84% had used cocaine, 82% alkylnitrites, 64% amphetamines, 51% quaaludes, 41% barbiturates, and 20% injected drugs, and 13% shared needles (107). This group had been randomly selected from a list of homosexuals who had volunteered to be investigated for hepatitis B virus infection and to donate antisera to hepatitis B virus between 1978 and 1980.

Nitrite inhalants and possibly other drugs are preferred by male homosexuals as aphrodisiacs because they facilitate anal intercourse (105, 111, 113, 114). For example, an early CDC study that included 420 homosexual men found nitrite use far more frequent among homosexuals than among heterosexuals and correlating directly with the number of different homosexual partners (57). Surveys studying the use of nitrite inhalants in San Francisco found that among homosexual men 58% were users in 1984 and 27% in 1991 compared to less than 1% among heterosexuals and lesbians of the same age group (115).

The nitrites are directly toxic as oxidants of biological molecules such as hemoglobin, and are effective mutagens (101, 103). The National Institute on Drug Abuse reports correlations from 69% (116) to virtually 100% (101, 113) between nitrite inhalants and Kaposi sarcoma and pneumonia, which are diagnosed as AIDS in the presence of antibody to HIV (50, 51, 54). In view of this, a causal link between nitrite inhalants and Kaposi sarcoma and pneumocystis pneumonia in homosexuals was first suggested in 1982 by the CDC (57) and other investigators (56, 58). As a consequence, the sale of nitrite inhalants was banned by the U.S. Congress in 1988 (Public Law 100-690) (117, 118). The direct and indirect toxicity associated with the long-term use of other recreational drugs has been described elsewhere (103).

c. Anti-HIV Drugs. About 80,000 Americans and 120,000 persons worldwide with and without AIDS currently take the cytocidal DNA chain-terminator AZT (54) and an unknown number take other DNA chain-terminators such as ddI and ddC (71). AZT has been prescribed since 1987 to symptomatic (51, 70, 79, 119), and since 1990 to asymptomatic, carriers of HIV, including babies and hemophiliacs (80, 120), in an effort to inhibit HIV DNA synthesis (121). Thus, an unknown, but possibly high, percentage of the 30,000 Americans that currently develop AIDS per year (52) have used AZT prior to or after the onset of AIDS. For instance, 249 out of 462 HIV-positive, AIDS-free homosexual men from Los Angeles, included in the above survey (105), are on AZT or ddI (122).

Although AZT is an inhibitor of HIV DNA synthesis, it is not a rational medication for persons with antibodies to HIV for the following reasons: (i) There is no proof that HIV causes AIDS. (ii) Since no detectable RNA-dependent viral-DNA synthesis occurs, and since the number of infected cells remains stable once the virus is neutralized by antibodies (53, 54) only cell DNA with and without proviruses of HIV is terminated by AZT treatment. Further, since AZT cannot distinguish infected from uninfected cells, and only 1 in 500 T-cells is infected in AIDS patients and asymptomatic carriers (54, 84), it kills 500 uninfected cells for every infected cell. Thus, AZT is inevitably toxic, killing 500 times more uninfected than infected cells. (iii) In view of the hypothesis that HIV causes AIDS by killing T-cells (49, 51), it is irrational to overkill infected cells with AZT.

As expected from an inhibitor of DNA synthesis, many studies report AZT-mediated toxicity. Anemia, neutropenia, and leukopenia occur in 20-50%, with about 30-50% requiring transfusions within several weeks (70, 71, 123-125). Severe nausea from intestinal intoxication is observed in up to 45% (70, 71, 80) and severe muscle atrophy in 6-8% (70, 126-128). Acute hepatitis, insomnia, headaches, dementia, seizures, and vomiting are also reported effects of AZT (71). Lymphomas appear in about 9% within 1 year on AZT (69). AZT is also mutagenic and carcinogenic in mice (129, 130) and transforms cells in vitro as effectively as methylcholanthrene (131). AZT toxicity varies a great deal with the patient treated, due to differences in kinases involved in its uptake and in AZT metabolism (71, 121, 131, 132). All of these results explain Temin's profound observation that "... the drug generally becomes less effective after six months to a year...." (134).

Nevertheless, AZT is thought to have serendipitous therapeutic benefits based on the only placebo-controlled study of its effects on AIDS patients (70, 119). The study was sponsored by Burroughs Wellcome, the manufacturer of AZT (70, 119). In this study, T-cell counts were observed to increase from 4 to 8 weeks and then to decline to pretreatment levels. Above all, AZT was claimed to "decrease mortality" because only 1 out of 143 in the AZT-treated group died compared to 19 out of 135 in the placebo group.

However, 30 out of the 143 in the AZT group depended on multiple transfusions for survival from anemia, compared to only 5 out of the 135 in the placebo group. Since the number of subjects in the AZT group who would have died from anemia if untreated (30) was larger than the AIDS deaths and anemias of the control group combined (19 + 5), the claim of decreased mortality is not realistic (70, 119). Moreover, 66 in the AZT group suffered from severe nausea and 11 from muscle atrophy, compared to only 25 and 3 in the control group. The lymphocyte count decreased over 50% in 34% of the AZT group and in only 6% of the control. The study is further compromised by "concomitant medication" (70), the failure to consider the effects of recreational drug use and of patient-initiated randomizations of blinded AZT and placebo controls (135). The brief AZT-induced gain of T-cells may reflect compensatory hemopoiesis and random killing of pathogenic parasites (132) and the influence of concomitant medication (70).

In view of the inevitable toxicity of AZT, its popularity as an anti-HIV drug can only be explained by the widespread acceptance of the virus-AIDS hypothesis and the failure to consider the enormous difference between the viral and cellular DNA targets. This may also be the reason that long-term studies of AZT in animals compatible with human applications have not been published (71).

3. The Drug- Versus the Virus-AIDS Hypothesis

To distinguish between HIV and drugs as causes of AIDS, it is necessary to determine whether HIV carriers develop AIDS only when they use drugs, and whether HIV-free drug users develop AIDS indicator diseases.

A. Drug Use Necessary for AIDS in Presumed or Confirmed Carriers of HIV. (i) Epidemiological correlations suggest that nitrites are necessary for Kaposi sarcoma. (a) A 27- to 58-fold higher consumption of nitrites (111, 115) correlates with a 20-fold higher incidence of Kaposi sarcoma in male homosexuals compared to all other AIDS patients of the same age group (65). (b) Among male homosexuals, those with Kaposi sarcoma have used nitrite inhalants twice as often as those with other AIDS diseases (101). (c) During the last 6-8 years, the use of nitrite inhalants among male homosexuals decreased (e.g., from 58% in 1984 to 27% in 1991 in San Francisco) (115). In parallel, the incidence of Kaposi sarcoma among American AIDS patients decreased from a high of 37% in 1983 (136) to a low of 10% in 1990 (52). In fact, nitrites may be sufficient causes for these diseases, because there is no evidence that HIV was even present in any of these studies.

(ii) Specific correlations indicate that nitrites are necessary for AIDS. The first five cases diagnosed as AIDS in 1981, before HIV was known, were male homosexuals who had all consumed nitrite inhalants and presented with pneumocystis pneumonia and cytomegalovirus infection (137). Early CDC data indicate that, in 1981 and 1982, 86% of male homosexuals with AIDS had used oral drugs at least once a week and 97% occasionally (57, 138), and that every one of 20 Kaposi sarcoma patients had used nitrites (56). The National Institute on Drug Abuse reports correlations from 69% (116) to virtually 100% (101,113) between nitrite inhalants and Kaposi sarcoma and pneumonia. Again, drugs may have sufficed to cause these diseases, because HIV was not diagnosed (50, 51, 54).

(iii) The incidence of AIDS diseases among 297 HIV-positive, asymptomatic intravenous drug users over 16 months was three times higher in those who persisted than in those who stopped injecting drugs (139).

(iv) The T-cell count of 65 HIV-infected drug users from New York dropped over 9 months in proportion with drug injection-on average, 35%-compared to controls who had stopped (140).

(v) A placebo-controlled study, investigating AZT as AIDS prophylaxis in HIV-positive, AIDS-free 25- to 45-year-old male homosexuals and intravenous drug users, indicates that AZT induces diseases from within and without the AIDS definition (80). During 1 year of taking 500 mg of AZT per day, a group of 453 developed 11 AIDS cases, and a group of 457, taking 1500 mg of AZT per day, developed 14 cases. The placebo group of 428 developed 33 AIDS cases.

However, the price for the presumed savings of 22 and 19 AIDS cases with AZT was high, because 19 more cases of anemia, neutropenia, and severe nausea appeared in the 500-mg AZT group, and 72 more such cases appeared in the 1500-mg AZT group, than in the placebo group. This indicates cytocidal effects of AZT on hemopoiesis and on the intestines. Although these AZT-specific diseases were not diagnosed as AIDS, neutropenia generates immunodeficiency. Surprisingly, in view of its toxicity on leukocytes and red cells, a consistent loss of T-cells was not observed in this study. A recent study investigating AZT as AIDS prophylaxis observed leukopenia, e.g., T-cell depletion, in 82% within 1 to 1.5 years of AZT treatment (140a). The study is further compromised by the failure to report and to consider the recreational drug-use histories and the many AZT-treatment adjustments of the subjects analyzed.

(vi) Within 48 weeks on AZT, 172 (56%) out of 308 AIDS patients developed additional AIDS diseases, including pneumonia and candidiasis (125). This indicates that AZT induces AIDS diseases within less than 1 year, and thus much faster than the 10 years HIV is said to need to cause AIDS (54). Likewise, no therapeutic benefits were observed for 365 French (123) and 4 Norwegian AIDS (133) patients after 6 months on AZT.

(vii) The annual lymphoma incidence of AZT-treated AIDS patients was reported to be 9% by the National Cancer Institute and was calculated to be 50% over 3 years (69). The lymphoma incidence of untreated HIV-positive AIDS-risk groups is 0.3% per year and 0.9% per 3 years, derived from the putative average progression rate of 10 years from HIV to AIDS (54, 141,142) and the 3% incidence of lymphoma in AIDS patients (52). Thus, the lymphoma incidence is 30-50 times higher in AZT-treated than in untreated HIV-positive counterparts. In addition, "during the past three years [of AZT therapy] a progressive increase in the number of [AIDS] patients dying from lymphoma, ..." to a current level of 16%, was noted in 1991 in a group of 346 AIDS patients in London, most of whom were on AZT (143).

It is likely that the chronic levels of the mutagenic AZT, at 10-30 µM (500-1500 mg/person/day), were responsible for the lymphomas. The alternative proposal that HIV-induced immunodeficiency was responsible for the lymphomas (69) is unlikely, since cancers do not reflect a defective immune system (53, 144).

(viii) Ten out of 11 HIV-positive, AZT-treated AIDS patients recovered cellular immunity after discontinuing AZT in favor of an experimental HIV vaccine (145), indicating that AZT sufficed for immunodeficiency.

(ix) Four out of five AZT-treated patients recovered from myopathy 2 weeks after discontinuing AZT; two redeveloped myopathy on renewed AZT treatment (126).

(x) Four patients with pneumonia developed severe pancytopenia and bone marrow aplasia 12 weeks after the initiation of AZT therapy. Three out of four recovered within 4-5 weeks after AZT was discontinued (124), indicating that AZT was sufficient for pancytopenia.

b. Drug Use Sufficient for AIDS Indicator Diseases in the Absence of HIV. (i) Among intravenous drug users in New York, representing a "spectrum of HIV-related diseases," HIV was observed in only 22 out of 50 pneumonia deaths, 7 out of 22 endocarditis deaths, and 11 out of 16 tuberculosis deaths (66).

(ii) Pneumonia was diagnosed in 6 out of 289 HIV-free and 14 out of 144 HIV-positive intravenous drug users from New York (146).

(iii) Among 54 prisoners with tuberculosis in New York State, 47 were street-drug users, but only 24 were infected with HIV (67).

(iv) In a group of 21 heroin addicts, the ratio of helper to suppressor T-cells declined within 13 years from a normal of 2 to less than 1, which is typical of AIDS (50, 51), but only 2 were infected by HIV (147).

(v) Thrombocytopenia and immunodeficiency were diagnosed in 15 intravenous drug users on average 10 years after they became addicted, but 2 were not infected with HIV (148).

(vi) Lymphocyte reactivity and abundance was depressed by long-term injection of drugs not only in 111 HIV-positive but also in 210 HIV-free intravenous drug users from Holland (149).

(vii) The same lymphadenopathy, weight loss, fever, night sweats, diarrhea, and mouth infections were observed in 49 out of 82 HIV-free and 89 out of 136 HIV-positive, long-term intravenous drug users from New York (150), and in about 40% of 113 intravenous drug users from France, of which 69 were HIV-positive and 44 were negative (151). The French group had used drugs for an average of 5 years.

(viii) Among six HIV-free male homosexuals with Kaposi sarcoma, five reported the use of nitrite inhalants (152).

(ix) Similar neurological deficiencies were observed among 12 HIV-infected and 16 uninfected infants from drug-addicted mothers (153).

Thus, the long-term use of recreational and anti-HIV drugs appears necessary in HIV-positives and sufficient in HIV-negatives to induce AIDS indicator and other diseases.

It follows that the drug-AIDS hypothesis is epidemiologically and pathologically better grounded than the virus-AIDS hypothesis. About 32% of American AIDS patients are confirmed intravenous drug users, probably 60% use aphrodisiac drugs orally, and an unknown but large percentage of both behavioral and clinical AIDS-risk groups use AZT. Moreover, the consumption of recreational drugs by AIDS patients is probably underreported, because the drugs are illicit, and because medical scientists and support for research are currently heavily biased in favor of viral AIDS (68, 154, 155). The pathogenicity of these drugs is empirically known for all, and mechanistically for some, drugs, notably for AZT and nitrites (103).

Nonetheless, evidence for the role of drugs in AIDS is rejected by proponents of the virus-AIDS hypothesis (15, 77, 105). This is certainly one reason why despite the current drug-use epidemic, there are no studies that investigate the long-term effects of psychoactive drugs and AZT in animals, compatible with the time periods and dosages used by AIDS patients (155).

By contrast to the near complete correlation between drugs and AIDS, antibodies to HIV are confirmed in only about 50% of AIDS patients (51, 72), and it is a complete mystery how HIV acts as a pathogen, despite enormous research efforts (14, 15, 54, 156).

The drug-AIDS hypothesis resolves all scientific paradoxes posed by the prevailing virus-AIDS hypothesis:

1. In America, HIV is a long-established, endemic virus, but AIDS is new-because the drug epidemic is new.

2. AIDS is restricted for over 10 years to 10,000 (52) or 0.01% of the over 100 million sexually active heterosexual Americans per year, and to 20,000 (52) or 0.25% of the 8 million homosexuals, estimated at 10% of the adult male population (109, 111). But conventional venereal diseases are on the rise in the U.S. (97), and there is no vaccine or drug against HIV. This is because AIDS is due to drug consumption rather than sexual activity.

3. Over 72% of American AIDS cases are 20- to 44-year-old males (52)-although no AIDS disease is male-specific (50, 51)-because males of this age group consume over 80% of all "hard" psychoactive and aphrodisiac drugs (101, 103, 111, 115, 157, 158).

4. Distinct AIDS diseases occur in distinct risk group-because they use distinct drugs (e.g., users of nitrites get Kaposi sarcoma, users of intravenous drugs get tuberculosis, and users of AZT get leukopenia and anemia).

5. Viral AIDS occurs, on average, 10 years after HIV infection (51, 53, 54), although infectious agents, being self-replicating toxins, typically strike within weeks or months after infection (11, 12). Indeed, HIV is immunogenic, and may be mildly pathogenic in humans within weeks after infection and is then "effectively and rapidly limited" by antiviral immunity (91, 92). This is because HIV infection and AIDS are unrelated events. The duration and toxicity of drug consumption and individual thresholds for disease determine when AIDS occurs, irrespective of when and whether HIV infects.

6. HIV, as well as many other parenterally and venereally transmitted microbes and viruses, are mere markers for AIDS and AIDS risks (54, 107, 159)-because the higher the consumption of unsterile, injected drugs (140, 151) and sexual contacts mediated by aphrodisiac drugs, the more microbes are accumulated.

7. Some old diseases of hemophiliacs, other recipients of transfusions, and the general American population are called AIDS-if they coincide with perinatal or parenteral HIV infection (54).

8. Old African diseases such as slim disease, fever, diarrhea, and tuberculosis are called AIDS now, although they are clinically and epidemiologically very different from American AIDS. They occur in adolescents and adults of both sexes that are subject to protein malnutrition, parasitic infections, and poor sanitary conditions (53). Only because HIV is endemic in over 10% of Central Africans are over 10% of old African diseases now called AIDS (51, 53, 54).

The drug-AIDS hypothesis predicts that the AIDS diseases of the behavioral AIDS-risk groups in the U.S. and Europe can be prevented by controlling the consumption of recreational and anti-HIV drugs, but not by "safe sex" (51) and "clean injection equipment" (55) for unsterile (!) street drugs. According to the drug-AIDS hypothesis, AZT is AIDS by prescription. Screening of blood for antibodies to HIV is superfluous, if not harmful, in view of the anxiety that a positive test generates among the many believers in the virus-AIDS hypothesis and the toxic AZT prophylaxis, prescribed to many who test "positive." Eliminating the test would also reduce the cost of the approximately 12 million annual blood donations in the U.S. (82) by $11 each (Irwin Memorial Blood Bank, personal communication, 1990) and would lift travel restrictions for antibody-positives to many countries, including the U.S. and China. The drug-AIDS hypothesis is testable epidemiologically and experimentally by studying AIDS drugs in animals.

B. Hepatitis C Virus and Non-A Non-B Hepatitis

Non-A non-B hepatitis is observed primarily in recipients of transfusions and in intravenous drug users (3, 12, 160). It has been postulated to be a viral disease because inoculation of plasma or serum (3-75 ml) from hepatitis patients into chimpanzees induced some biochemical markers of hepatitis, such as alanine aminotransferase, in half of the animals (160). However, none of the animals developed hepatitis (161, 162). Trace amounts of presumably viral RNA have recently been detected in the liver of hepatitis patients. In addition, "nonneutralizing" antibodies to "nonstructural epitopes," from an apparently latent RNA virus, have been identified mostly in asymptomatic carriers (160). Cloning and sequencing indicated that the RNA is directly coding and measures about 10 kb. Therefore, the suspected virus has been tentatively classified as a togavirus (160). Viral RNA was only detectable after amplification with the PCR in 9 out of 15 non-A non-B hepatitis patients, and non-neutralizing antibodies were found in only 7 of the 9 RNA-positive and in 3 of the 6 RNA-negative patients (163). Likewise, liver tissues from chimpanzees inoculated with sera from hepatitis patients contain only one viral RNA molecule per ten cells (160).

In view of this evidence, the putative virus has been termed hepatitis C virus (HCV) to indicate that it is the cause of the hepatitis. Subsequently, the Food and Drug Administration has recommended, and the American Association of Blood Banks has mandated, as of 1990, the testing of the approximately 12 million annual blood donations in the U.S. (82) for antibodies to HCV at an approximate cost of $5 per test. The test was developed by Chiron Co., Emeryville, California (Irwin Memorial Blood Bank, personal communication, August 15, 1991).

However, several arguments cast doubt on the hypothesis that HCV causes hepatitis:

1. Virus-containing sera or plasma from hepatitis patients does not cause hepatitis if inoculated into chimpanzees, indicating that HCV is not sufficient to cause the disease. Moreover, since the virus has not been propagated in culture and isolated in a pure form, the possibility exists that the biochemical markers of hepatitis that are observed in chimpanzees inoculated with plasma were induced by another agent. Thus, HCV is not likely to be a sufficient cause of hepatitis in humans.

2. The presence of HCV in asymptomatic subjects at the same concentration and activity as in hepatitis patients also indicates that the virus is not sufficient to cause hepatitis.

3. The absence of viral RNA in 6 out of 15 hepatitis C patients indicates that the virus is not necessary for the disease.

It appears that HCV either causes disease by unprecedented mechanisms with as little as one RNA molecule per 10 liver cells in some and even less in other carriers, or that the virus is not the cause of non-A non-B hepatitis. By contrast, the concentration of viral RNAs made by conventional pathogenic viruses, including togaviruses, ranges from 103 to over 104 per cell (10). Therefore, it seems plausible that a latent passenger virus was identified that survives by establishing chronic asymptomatic infections at very low, nonpathogenic titers (164).

C. Measles Virus, HIV, and Subacute Scleroting Panencephalitis

In 1967, a cytocidal measles virus was proposed to be the cause of a very rare, subacute scleroting panencephalitis of children (165), based on correlations with antibodies to the virus or trace amounts of virus (3, 10, 12). The encephalitis is observed only 1-10 years after an acute primary infection, in the face of antiviral immunity, and in only about 1 out of 1 million children infected by the virus (3, 10, 12). The virus can only be isolated from the brains of 2 out of 8 encephalitis patients after cocultivation of brain cells with susceptible human cells (166). Thus, only a few intact virus particles are present in the brains of some, but apparently not in all, children with encephalitis. Viral gene expression in brain autopsies is 10- to 200-fold lower than in virus-replicating control cells, amounting to as few as 10 mRNAs per cell (167). Moreover, mutations and deletions were observed in these viral RNAs compared to wild-type measles virus (168). Accordingly, some viral RNAs are not even translated (3). By contrast, the wild-type virus causes measles within weeks after infection, at very high virus titers, and prior to antiviral immunity (10, 12).

The measles virus-encephalitis hypothesis has a number of epidemiological and virological shortcomings:

1. Since the disease does not occur concurrently with, or instead of, the conventional measles disease during a primary infection, and since antiviral immunity does not protect against the disease, measles virus cannot be sufficient to cause the subacute panencephalitis.

2. The virus cannot be a sufficient cause of the disease because only 1 in 106 infected persons develops panencephalitis, compared to one in a few if not all who develop measles disease before antiviral immunity (3, 10, 12).

3. Since viruses are self-replicating toxins, all are potentially "fast" pathogens, but encephalitis is observed only 1-10 years after infection, measles virus cannot be sufficient for panencephalitis.

4. The absence of infectious virus in some panencephalitis cases, and the very low concentration of viral RNA in all cases, suggest that measles virus is either not causative, or is causative by a mechanism that is totally different from that causing measles disease. During conventional measles disease, the virus is abundant, making over 1000 RNA molecules per cell in large numbers of cells (3, 10, 12, 167, 168).

In view of these paradoxes, it was suggested that selection of viral mutants would account for the encephalitis-pathogenicity of the virus (3, 167, 168). However, this seems unlikely, because the virus does not replicate sufficiently in encephalitis patients to generate new pathogenic variants, and because natural variants with a neurotropic specificity would then be expected.

About 15 years after the measles virus-encephalitis hypothesis was advanced, others proposed that the encephalitis was caused by a latent retrovirus closely related to HIV (169). This hypothesis also suffers from the problem that the presumed viral pathogen is latent (169). In addition, an encephalopathy is hard to reconcile with the fact that retroviruses depend on mitosis for infection (170) and the fact that neurons stop dividing soon after birth (1).

D. Phantom Viruses and Neurological Disease

The strong belief in viruses as causes of diseases has in some instances even exceeded their very definition. For example, the Nobel Prize in 1976 was given for hypothetical, slow, and unconventional viruses that would cause neurological diseases such as kuru, Creutzfeld-Jacob's, and Alzheimer's diseases, after long latent periods of up to 30 years (171). Kuru is a now-extinct neurological disease of a small tribe of 35,000 in New Guinea that reportedly was transmitted by ritual cannibalism (3, 12, 171). "Slow and unconventional" viruses have been postulated because 4 out of 7 chimpanzees had developed neurological diseases about 1-2 years after they had been inoculated intracerebrally with brain suspensions from kuru patients (172). The presumed Creutzfeld-Jacob virus failed to induce neurological disease if presumably infected materials were inoculated into the brains of chimpanzees (3). A slow, unconventional virus has also been claimed as the cause of scrapie, a neurological disease of sheep (3, 12).

Since the incubation periods from inoculation of brain suspensions from kuru patients to neurological disease in the animals (1-2 years) and from presumed infection of humans to kuru (up to 30 years) differ significantly, it is not clear whether the diseases were caused by the same agent. Considering the claim that the viruses are naturally transmitted by cannibalism, it seems inappropriate that the traumatic intracerebral inoculation was chosen to test the oral transmission hypothesis. Nevertheless, Gajdusek et al. pointed out, "To anyone who had the opportunity of observing the unique syndrome of kuru ... the similarity of its clinical picture and course to the experimentally induced syndrome ... is dramatically evident" (172).

The slow virus-neurological disease hypothesis suffers from several shortcomings:

1. None of these hypothetical viruses has ever been isolated and chemically analyzed. Their presumed properties all far exceed the known ranges of conventional viruses and even of known proteins and nucleic acids. For example, the kuru and Creutzfeld-Jacob viruses are said to resist boiling water, ionizing gamma radiation, ultraviolet radiation, and inactivation with formaldehyde (3, 171 ). Moreover, the viruses are not antigenic, and not visible under the electron microscope, although available preparations are reported to have titers of 107 lethal doses per milliliter (3). Paradoxically, the slow, unconventional viruses have since evolved into an infectious protein, termed prion, "derived from a normal cellular protein ... through an unknown posttranslational process" (173).

2. The virus-kuru hypothesis fails to account for the long latent periods between presumed infection and disease and for the restriction of the disease to a very specific risk group.

3. A recent analysis of the original data on kuru transmission casts doubt on the virus-kuru hypothesis, because the evidence for cannibalism was fabricated (174).

In view of this, we agree with a review by Gibbs, a collaborator of Gajdusek, that "many paradoxes [were] thrust on us by the discovery of these unconventional viruses as the etiological agents of chronic, progressive, degenerative diseases of the central nervous system . . . " and that "toxic or genetic determinants and even trauma lead to the same pathogenesis ..." (3). Indeed, it seems plausible that the toxicity and trauma of intracerebral inoculations of human brain suspensions from kuru patients could cause neurological diseases without phantom viruses said to be the etiological agents. The restriction of the slow neurological diseases to specific ethnic groups or to sporadic cases could reflect genetic and acquired deficiencies rather than selective and slow viruses.

III. Viruses as Causes of Clonal Cancer

A. Human T-cell Leukemia Virus and Adult T-cell Leukemia

Human T-cell leukemia virus-I (HTLV-I) was originally discovered in a T-cell line from a leukemic patient (175). This line, termed HUT 102, only produced virus after it had been propagated in vitro, in the absence of the virus-suppressing immune system of the host, and after it had been treated with mitogens and mutagens such as iododeoxyuridine, an agent known to activate dormant retroviruses (6). Since the virus was isolated from a cell line that came from an adult patient with T-cell leukemia, the virus was proposed to be the cause of adult T-cell leukemia (ATL), and hence named human T-cell leukemia virus (175, 176). However, a parallel T-cell line, termed HUT 78, derived from another patient with T-cell leukemia, failed to yield a retrovirus (87).

Further support for the hypothesis was derived from epidemiological correlations between antibodies to HTLV-I and ATL in Japan and the U.S. (3, 37, 176). Based on 30,000 blood donations, the American Red Cross has reported that in 1986-1987 about 0.025% or 65,000 Americans were infected with HTLV-I (3, 82), but the American T-cell Leukemia/Lymphoma Registry had recorded in 1990 in the U.S. no more than 90 ATLs. Among these, 75 were non-Caucasians (177), a group in which HTLV-I is often endemic (178). However, the same Registry also reports, "although most cases of ATL are HTLV-I-associated ... many are not" (177). As in the U.S., HTLV-I-free ATLs have been observed in Japan (1 79). A controlled study comparing the incidence of the leukemia in HTLV-I-positive and -negative control groups has never been published.

By definition, "The diagnosis of ATL is made from the characteristic clinical findings, the detection of serum antibodies to HTLV-I and, when necessary, the confirmation of monoclonal integration of HTLV-I proviral DNA in cellular DNA of ATL cells" (180). According to this tautology, ATL is defined and distinguished from virus-free T-cell leukemias solely by the presence of antibody to HTLV-I or viral DNA.

In addition, HTLV-I is also postulated to cause an HTLV-I-associated myelopathy (HAM), which is a neurological disease also defined only by the presence of HTLV-I (3, 181).

ATL is clonal, originating from a single cell, like virus-free T-cell leukemias. The clonality of the leukemia is defined by chromosome abnormalities, as well as by clonal proviral integration sites (2, 176). However, there are no specific integration sites of HTLV-I in different leukemias (2). In leukemic cells, the virus is always latent, suppressed by antiviral immunity, and sometimes even defective (2). It is for this reason that the virus was originally discovered only in vitro, after reactivation from latently infected leukemic cells grown in culture.

HTLV-I, like other non-oncogenic retroviruses (6, 54), is naturally transmitted from mother to child with an efficiency of 22% based on testing for antiviral antibodies (176, 182, 183). Indeed, latent proviruses appear to be transmitted perinatally at a higher efficiency than antibody tests indicate, because the antibody titers increase with age (176) at a much faster rate than could be accounted for even by thousands of sexual contacts (183). Thus, this virus, like all other retroviruses without oncogenes (54), survives from perinatal transmission. Sex is another, although highly inefficient, mode of transmission, depending on an average of over 1000 sexual contacts (183).

Based on epidemiological studies from Japan, HTLV-I is said to cause leukemia in only 1-5% of all virus carriers in a lifetime (182). The annual incidence of the leukemia per HTLV-I carrier in Japan is estimated to be only 1 in 1000 (182, 184). Since HTLV-I is a perinatally transmitted retrovirus, but leukemia typically appears, if at all, only in 50- to 60-year-olds, the latent period from infection to disease is estimated at 55 years (176, 185).

The following epidemiological and virological arguments cast doubt on the HTLV-I-leukemia hypothesis:

1. According to the American Red Cross, "ATL ... as of September 1989, has not been reported in association with transfusion transmitted HTLV-I infection," although about 65,000 Americans were infected with HTLV-I and about 12 million blood donations are annually transfused to millions of recipients in the U.S. (82). Thus, HTLV-I cannot be sufficient to cause leukemia.

2. Since viruses, as self-replicating toxins, are all potentially fast pathogens, but leukemia is only observed about 55 years after infection, HTLV-I cannot be sufficient for leukemia.

3. Considering that 1% of HTLV-I carriers develop ATL per lifetime in Japan and about 0.1% (90 : 65,000) in the U.S., that the leukemias are clonal deriving from single cells, and that each carrier must contain at least 107 latently infected T-cells (because the limit of provirus detection by hybridization is 1 in 1000 cells) and that humans contain 1010 to 1011 T-cells that go through at least 420 generations in a 70-year lifetime (see Section IV) (37, 186), then only 1 out of 102 (Japan) to 103 (U .S.) x 107 x 420 = 1012 infected T-cells become leukemic. Thus, HTLV-I cannot be sufficient for leukemogenesis.

4. Antiviral antibodies that completely neutralize HTLV-I to virtually undetectable levels (2) do not protect against the leukemia. This also indicates that HTLV-I is not sufficient for leukemogenesis.

5. Retroviruses cause either polyclonal tumors via dominant, biochemically active oncogenes (6, 37), or possibly clonal tumors via site-specific integration that generates active virus-cell hybrid oncogenes (31, 40, 42). Yet HTLV-I neither expresses a leukemia-specific gene product that could function as an active oncogene, nor does it integrate at a specific site in different "viral leukemias" (2, 187). Thus, HTLV-I cannot be sufficient for leukemogenesis.

6. The statement of the American T-cell Leukemia/Lymphoma Registry that "although most cases of ATL are HTLV-I associated ... many are not" (177) and the reports of virus-negative leukemias from Japan (179) and other countries (2) indicate that HTLV-I is not even necessary for the disease.

7. The HTLV-I-leukemia hypothesis fails to explain the clonal chromosome abnormalities that are consistently found in all ATLs (2, 188)-except if one makes the additional odd assumption that HTLV-I only transforms cells with a preexisting chromosome abnormality.

Thus, there are no virus-determined diagnostic criteria, besides the presence of antiviral antibodies, nor are there any controlled epidemiological and virological criteria to support the hypothesis that HTLV-I is the cause of ATL. Therefore, ad hoc hypotheses have been advanced proposing "a second oncogenic event, such as a chance translocation or a second oncogenic virus ..." for viral leukemogenesis (187). Others estimate five steps in leukemogenesis, of which HTLV-I is postulated to be an "initiator" (185).

Since not even one transfusion-transmitted leukemia case has been recorded in the U.S., it seems surprising that a blood test for antibodies against HTLV-I has become mandatory for members of the American Association of Blood Banks since February 1989. It raises the cost of each of the approximately 12 million annual blood donations in the U.S. (82) by $5-11 (189; Irwin Memorial Blood Bank, personal communication, 1990). Indeed, an HTLV-I epidemiologist pointed out, "Ironically, this route of [HTLV-I] transmission is numerically the least important," considering the 55-year average latent period from infection to leukemia, "the advanced age of most U.S. blood recipients, and the observation that as many as 60% of transfusion recipients may die within approximately 3 years of transfusion because of their underlying disease" (183). Nevertheless, in terms of blood testing expenses, HTLV-I has reached cost-parity with HIV, which adds another $11 test fee to each blood donation (Irwin Memorial Blood Bank, personal communication, 1990).

An alternative hypothesis suggests that spontaneous or perhaps radiation-induced chromosome abnormalities induce the clonal leukemias (see Section VI). Nuclear radiation from the Hiroshima and Nagasaki bombs is blamed for 147 leukemias (190). By proposing that one out of billions of normal HTLV-I-infected cells is transformed by a spontaneous chromosome abnormality, our hypothesis readily resolves the paradox of the clonal chromosome abnormalities in all "viral" leukemias.

B. Herpes Virus, Papilloma Viruses, and Cervical Cancer

Inspired by the SV40/adenovirus-cancer models, infection by herpes simplex virus (HSV) was postulated in the 1970s to be the cause of cervical cancer based on epidemiological correlations with HSV DNA (3). The virus is sexually transmitted and is latent in about 85% of the adult population of the U.S. (3). Infection by intact HSV typically kills the cell. However, defective and intact viruses that become latent do not kill cells (3).

The viral DNAs in cervical cancers are defective and integrated with cell DNA. Cervical cancers with defective HSV DNA are clonal, just like virus-free cancers (191-194). In agreement with the SV40/ adenovirus models, HSV does not replicate in the tumors. But, unlike the SV40/adenovirus models, no set of viral genes is consistently present or expressed in human cervical cancers. Therefore, the "hit-and-run" mechanism of viral carcinogenesis was proposed (195). It holds that neither the complete HSV, nor even a part of it, needs to be present in the tumor. Obviously, this is an unfalsifiable, but also an unprovable, hypothesis.

Also inspired by the SV40/adenovirus models, and based on epidemiological correlations, infection by human papilloma virus (HPV) was postulated in the 1980s by zur Hausen to be a causative factor in cervical and anogenital cancers (3, 191, 196).

Papilloma viruses are transmitted by sexual and other contacts, like the herpes viruses, and are widespread or "ubiquitous" in at least 50% of the adult population of the U.S. and Europe (3, 191). For example, using the PCR to amplify sequences of one particular strain of papilloma virus, 46% of 467 women in Berkeley, California, with a median age of 22 were found to carry HPV, but none of them had cervical cancer (199). Many other strains of HPV exist (3, 191) that could not be detected with the assay used in this study (199). Like the SV40/ adenovirus models, HPV does not replicate in the tumors. But, unlike these models, HPV naturally replicates nonlytically (13), forming polyclonal warts with unintegrated viral DNA plasmids (200).

zur Hausen reports that cervical cancers occur in less than 3% of infected women in their lifetime, but the incidence in HPV-free controls was not reported (191). In the U.S., the incidence of cervical cancer in all women, with and without HPV, per 70-year lifetime is about 1% (197). In a controlled study of age-matched women, 67% of those with cervical cancer and 43% of those without were found to be HPV-positive (198). These cancers are observed on average only 20-50 years after infection (191).

Different sets and amounts of viral DNA are integrated into cell DNA of different carcinomas (191), and viral DNA is poorly expressed in some cancers and not expressed at all in others (3, 191, 201). Moreover, different HPV strains are found in different cancers (3, 191, 196). Viral antigens are found in only 1-5% of carcinomas (3). Accordingly, HPV does not replicate in the cancer cells and there are no reports of HPV-specific histological or physiological markers that set HPV DNA-positive apart from negative carcinomas (191). There is also no virus-specific integration site in HPV DNA-positive cancers (191), indicating that no specific cellular gene is activated, or that a tumor suppressor gene is inactivated by integration of viral DNA. HPV DNA-positive tumors are clonal and carry clonal chromosome abnormalities, just like virus-negative tumors (191-194).

The HPV-cancer hypothesis of zur Hausen proposes that HPV encodes a "transforming factor" that is suppressed in normal cells by a cellular interference factor (CIF). Inactivation of both CIF alleles by mutation is postulated to result in viral carcinogenesis (191). The low probability of developing mutations in both suppressor alleles is said to explain the long intervals between infection and cancer. This hypothesis correctly predicts that only a small fraction of infections lead to cancer. It further predicts clonal tumors with active HPV DNA and mutations in both alleles of the suppressor genes, and it predicts no effects on the karyotypes of cells.

Howley et al. proposed that a viral protein neutralizes the proteins of the retinoblastoma and p53 tumor suppressor genes, and that neutralization of these suppressor proteins causes cancer (202). The proposal is modeled after the hypothesis that retinoblastoma is caused by a cellular cancer gene, provided that a complementary suppressor gene, termed the retinoblastoma gene, is inactivated (see Section IV). This hypothesis predicts polyclonal tumors.

The following epidemiological and biochemical arguments cast doubt on these HPV-cancer hypotheses:

1. Random allelic mutation of suppressor genes, as postulated by zur Hausen, predicts a few cancers soon, and more long after infection. Since cancers only appear 20-50 years after infection, cooperation between HPV and mutations cannot be sufficient for carcinogenesis.

2. Further, the proposal of zur Hausen that inactivation of host suppressor genes is necessary for viral transformation is not compatible with HPV survival. Since HPV, like all small DNA viruses, needs all of its 8-kb DNA for virus replication (13), suppression of one or more HPV proteins by normal cellular genes would effectively inhibit virus replication in all normal cells. Conversely, if viral transforming proteins were not suppressed by normal cells, virus-replicating wart cells should be tumorigenic because all viral genes are highly expressed in virus replication (1, 13, 191).

3. The clonality of cervical cancers rules out the Howley hypothesis.

4. The lack of a consistent HPV DNA sequence and of consistent HPV gene expression in HPV DNA-positive tumors is inconsistent with the zur Hausen and Howley hypotheses and indicates that HPV is not necessary to maintain cervical cancer.

5. The presence of HPV in no more than 67% of age-matched women with cervical cancer (198) also indicates that HPV is not necessary for cervical cancer.

6. The hypothesis also fails to explain the presence of clonal chromosome abnormalities consistently seen in cervical cancer (16, 192-194)-except if one makes the additional odd assumption that only cells with preexisting chromosome abnormalities are transformed by HPV.

It follows that neither HPV nor HSV plays a direct role in cervical carcinomagenesis. Moreover, the HPV-cancer hypothesis offers no explanation for the absence of a reciprocal venereal male carcinoma.

Thus, detecting inactive and defective viral DNA from past infections in non-tumorigenic cells with a commercial hybridization test (Vira/Pap, Digene Diagnostics, Silver Spring, Maryland) or with the PCR (199) seems worthless as a predictor of rare carcinomas appearing decades later, in view of the "ubiquity" (191) of these viruses in women and the total lack of evidence that cervical cancer occurs in women with HPV more often than in those without. This test, at $30-150, is currently recommended for the 7 million Pap smears that appear "atypical" in the U.S. per year (Digene Diagnostics, personal communication, 1991). By contrast only 13,000 cervical cancers are observed annually in both HPV-positive and -negative women in the U.S. (197). Indeed, the test may be harmful, considering the anxiety a positive result induces in believers of the virus-cancer hypothesis.

An alternative cervical carcinoma hypothesis suggests that rare spontaneous or chemically induced chromosome abnormalities, which are consistently observed in both HPV and HSV DNA-negative and -positive cervical cancers (192-194), induce cervical cancer. For example, smoking has been identified as a cervical cancer risk (204). The controlled study of age-matched women described above suggests that 52% of the women with cervical cancer were smokers compared to only 27% of those without (198). Indeed, carcinogens may be primary inducers of abnormal cell proliferation rather than HPV or HSV. Since proliferating cells would be more susceptible to infection than resting cells, the viruses would be just indicators, rather than causes of abnormal proliferation. Activation of latent retroviruses like HTLV-I (Section III,A) (2), herpes viruses (12), and lambda phages (205) by chemical or radiation-induced cell damage and subsequent proliferation are classical examples of such indicators. Indeed, Rous first demonstrated that the virus indicates hydrocarbon-induced papillomas; it "... localized in these and urged them on ..." and suggested that enhanced proliferation is a risk factor for carcinogenesis (203).

According to this hypothesis, HPV or HSV DNAs in tumor cells reflect defective and latent viral genomes accidentally integrated into normal or hyperplastic cells, from which the tumor is derived. This hypothesis readily reconciles the clonal chromosome abnormalities with the clonal viral DNA insertions of the "viral" carcinomas. The inactive and defective viral DNA in the carcinomas would be a fossil record of a prior infection that was irrelevant to carcinogenesis.

C. Hepatitis B Virus and Liver Carcinoma

Epidemiological evidence indicates that chronic hepatitis B virus (HBV) carriers in Asia have a 250-fold higher risk of developing hepatomas than do non-carriers (3, 12, 206-208). The virus is typically transmitted perinatally in Asia and Africa (3, 207). In over 95% of infections in Asia and 99.9% in the U.S. and Europe the virus is completely neutralized by antiviral immunity. In people with drug- or disease-induced immunodeficiencies the virus remains chronically active (12). Approximately 1 out of 70 chronic HBV carriers in Asia develop clonal hepatomas and 1 out of 300 develop liver cirrhosis in their lifetime (3, 207). However, the liver tumors appear only in 30- to 60-year-olds. Moreover, chronic HBV carriers in Asia are "more likely" to develop hepatomas than those in Europe and the U.S. (12). Inoculation of HBV into chimpanzees has failed to cause hepatomas (3).

The virus is thought not to kill infected cells and viral DNA is replicated as a plasmid and thus not typically integrated into the host DNA (3, 12). However, molecular studies have detected clonal inserts of HBV DNA randomly integrated into the cellular DNA of liver carcinoma tissues (196, 209). Viral DNA is defective and not replicated in HBV DNA-positive hepatomas (209), like SV40 and adenovirus DNAs in the corresponding viral tumors. By contrast to the SV40/adenovirus models, no subset of viral DNA is consistently found or expressed in HBV-positive tumors (209, 210). Only 11-19% of tumors in HBVpositive patients express some viral antigens, compared to 26-61% expressing them in surrounding non-tumorous tissues (211). In addition to clonal inserts of HBV DNA, the hepatomas carry clonal chromosome abnormalities (16, 193, 196).

On the basis of these data, it has been proposed that HBV causes liver carcinoma in a step-wise process that begins with antigenemia, followed by chronic hepatitis, cirrhosis, and cancer (3, 207, 209). However, cirrhosis is not a necessary precursor of a hepatoma (3).

The following epidemiological and biochemical arguments cast doubt on the HBV-hepatoma hypothesis:

1. The long intervals of 30-60 years between infection and hepatomas indicate that HBV is not sufficient to initiate carcinogenesis.

2. The evidence that HBV is naturally transmitted perinatally also indicates that the virus is not sufficient to cause fatal diseases such as cirrhosis and hepatomas, because the viruses that depend on perinatal transmission for survival are not inherently pathogenic.

3. The evidence that the hepatoma risk among chronic HBV carriers in Asia is higher than in the U.S. and Europe also indicates that HBV is not sufficient for carcinogenesis.

4. The clonality of the HBV-positive hepatomas further indicates that HBV is not sufficient for carcinogenesis, because only one out of billions of chronically infected liver cells becomes tumorigenic.

5. The absence of an HBV-specific tumor marker, and of a specific HBV DNA sequence or integration site in viral hepatomas, both indicate that HBV is not necessary to maintain hepatomas.

6. The HBV-hepatoma hypothesis fails to explain the clonal chromosome abnormalities of hepatomas-except if one makes the additional odd assumption that HBV only transforms cells with preexisting chromosome abnormalities.

Thus, there is no convincing evidence that HBV DNA is functionally relevant for the initiation and maintenance of hepatomas. Its presence in the tumor could merely reflect that the tumor had originated from one of probably many liver cells of HBV carriers that contain defective, biochemically inactive viral DNA integrated randomly into their chromosomes (196). Therefore, molecular analysis of HBV DNA and of HBV DNA integration sites (210) is not likely to illuminate carcinogenesis.

However, chronically replicating HBV may function as an indirect carcinogen in the form of a long-term source of intoxication, inducing necrosis and tissue regeneration, a known risk factor for carcinogenesis (1, 196, 203). This view is consistent with the higher-than-normal incidence of hepatomas in persons with chronic HBV infection.

A competing hypothesis suggests that chronic HBV infection may only be an indicator of a chronic nonviral intoxication and immunodeficiency. Indeed, nonviral factors are involved in hepatomagenesis because the incidence of the hepatomas per HBV carrier varies with different countries (12). Intoxication could induce tissue regeneration and immune suppression, a classical precondition for opportunistic virus infections (see HPV in Section III, B). According to this view, the hepatoma would be caused by a rare virus-independent mechanism that generates chromosome abnormalities in one of many normal cells with HBV DNA inserts. This hypothesis would readily resolve the presence of the clonal chromosome abnormalities in all "viral" hepatomas. The defective and inactive viral DNAs in the hepatomas would be a fossil record of a prior infection that was irrelevant to carcinogenesis.

D. Epstein-Barr Virus and Burkitt's Lymphoma

In the early 1960s, Burkitt suggested that a B-cell tumor, now called Burkitt's lymphoma, which occurs in 1 out of 10,000 Central African children per year between 4 and 14 years of age, was caused by a virus (3, 12). Although not detectable in biopsies of lymphoma patients, a virus was found with the electron microscope in lymphoma cells grown in culture away from the suppressive immune system of the host (212). The Epstein-Barr virus (EBV) has since been postulated to be the cause of Burkitt's lymphoma (3, 8, 12).

In Central Africa, infection with the virus occurs perinatally in the first months of life in almost 100% of the population (3, 207). In the U.S. and Europe, infection occurs typically during or after puberty in about 50% of healthy adults (3, 213). However, the incidence of lymphomas with EBV in these countries is only less than 1 in 106 per year (3). Moreover, only 30% of otherwise indistinguishable lymphomas express EBV antigens (3). In America, Burkitt's lymphomas free of EBV DNA were described in 1973 (214). In China, EBV is also said to cause nasopharyngeal carcinoma in adults (1, 3).

During a primary infection, the virus may induce transient, polyclonal lymphoproliferative diseases, such as mononucleosis, if a large percentage of lymphocytes are infected prior to immunity. After antiviral immunity is established, the virus remains chronically associated with the host in a latent form (3, 12). During the chronic state of infection, viral DNA is detectable with the PCR in about 1 out of 105 lymphocytes (213) and viral antigens in only about 1 out of 107 lymphocytes (12).

In lymphomas, the virus is also suppressed, producing but a few viral antigens (3), as the history of its discovery had first indicated. Burkitt's lymphomas are clonal, deriving from single cells that carry characteristic chromosome translocations that often rearrange the proto-myc gene (see Section IV). Since EBV, like other herpes viruses, generally does not integrate into the host chromosome (1, 3), the time of infection of tumor cells (e.g., whether infection occured before, during, or after tumorigenesis) cannot be determined.

The EBV-lymphoma hypothesis suffers from numerous epidemiological and biochemical inconsistencies:

1. The clonality of the lymphomas that emerge from a single tumorigenic cell among billions of non-tumorigenic EBV-infected cells indicates that EBV is not sufficient for tumorigenesis.

2. The long intervals between infection and carcinogenesis, averaging 10 years in Africa, and the incidence of only 1 lymphoma per 10,000 infected persons also indicate that EBV is not sufficient to initiate tumorigenesis.

3. The lymphoma incidence varies over 100-fold between African and European or American EBV carriers, also indicating that EBV cannot be sufficient to cause a lymphoma.

4. The lack of a lymphoma-specific EBV function in symptomatic carriers indicates that EBV is not necessary to maintain lymphomas.

5. The existence of EBV-free Burkitt's lymphomas in American and European patients indicates most directly that EBV is not even necessary for Burkitt's lymphoma.

Thus, EBV appears neither necessary nor sufficient for lymphomagenesis. Nevertheless, it has been argued that EBV plays at least an indirect role in lymphomagenesis, because only a minority of susceptible cells from EBV-positive patients are infected in vivo, but virtually all lymphoma cell lines in culture are infected by the virus (215, 216). However, this could be an artifact of studying cells in culture, because the virus would spread unimpaired by immunity from a few infected, normal, or lymphoma cells to all lymphoma cells that survive in culture.

Since about 100% of the Central African and 30-50% of the American population carries latent EBV, and since EBV-negative Burkitt's lymphomas exist, it is likely that the correlations between EBV and tumors are accidental rather than causal. In view of this, an alternative hypothesis has been advanced, which holds that altered cellular proto-myc genes are the cause of Burkitt's lymphoma (see Section IV).

IV. Mutated Oncogenes, Anti-oncogenes, and Cancer

A. Mutated Proto-myc Genes and Burkitt's Lymphoma

The transforming gene of the directly oncogenic avian carcinoma virus MC29 contains a specific coding region, now termed myc (217), derived from a cellular gene termed proto-myc (218). Thus, the viral myc gene is a genetic hybrid that consists of a strong retroviral promoter linked to a coding region that is a hybrid of virus- and proto-myc derived sequences (219). This viral myc gene, like synthetic hybrids in which the native proto-myc promoter is replaced with that of a retrovirus (40, 42), is expressed to about 100-fold higher levels in all virus-transformed cells in vitro and in viral tumors than the cellular proto-myc genes (220-222).

The cellular proto-myc gene, located on chromosome 8, is rearranged with immunoglobulin genes from chromosomes 2, 14 and 22 in all (29) or most (30) cell lines derived from Burkitt's lymphomas. However, direct cytogenetic studies show that chromosome 8 is rearranged in only about 50% of primary Burkitt's lymphomas (223-226). Analogous rearrangements have also been observed in the proto-myc genes of mouse plasmacytoma cell lines (1, 8, 36). The rearrangements do not alter the coding region of proto-myc genes. Most rearrangements link the proto-myc coding regions to genetic elements from cellular immunoglobulin genes in the opposite transcriptional orientation (1, 8, 36). Other rearrangements in Burkitt's lymphomas do not affect the location and structure of proto-myc on chromosome 8, but instead rearrange regions 3' from proto-myc (36, 227-232). Because both retroviral myc genes and the rearranged proto-myc genes of most, but not all, Burkitt's lymphomas differ from normal proto-myc genes in truncations 5' from the coding region, and because both were found in cancers, the viral and rearranged cellular myc genes were proposed to be equivalent oncogenes (6, 8, 29, 30).

The transcriptional activity of the rearranged proto-myc genes in lymphomas is moderately enhanced, not altered, or even suppressed in Burkitt's lymphoma cells compared to normal proliferating cells (5, 30, 36, 216, 227). It is thus nearly 100-fold lower than that of viral myc genes or proto-myc genes artificially linked to retroviral promoters (40, 42, 220-222, 233).

Moreover, rearranged proto-rnyc genes from Burkitt's lymphomas do not transform any human or rodent cells upon transfection (5, 36, 38)-even if they are artificially linked to retroviral promoters (234, 236). In efforts to develop a system that is more efficient than transfection for introducing mutated proto-myc genes into cells or animals, synthetic avian retroviruses with the coding region of the human proto-myc gene were constructed (233, 237). Since these viruses transform avian cells, it was concluded that "ungoverned expression of the gene can contribute to the genesis of human tumors" (237). However, transformation of human cells was not demonstrated. Moreover, three independent studies report that murine cells cannot be transformed by authentic avian (238) and synthetic murine retroviruses with myc genes (239, 240), signaling a restricted transforming host range of myc genes.

Several arguments cast doubt on the hypothesis that rearranged proto-myc genes of Burkitt's lymphomas are functionally equivalent to retroviral myc genes and thus oncogenic:

1. Rearranged proto-myc genes from Burkitt's lymphomas or mouse plasmacytomas lack transforming function in transfection assays, while retroviral myc genes and proto-myc genes driven by retroviral promoters are sufficient to transform at least avian primary embryo cells (40, 42, 237). This indicates that the proto-myc genes from lymphomas and viral myc genes are functionally not equivalent.

2. Since expression of rearranged proto-myc genes from lymphomas is either the same as, or similar to, that of normal proto-myc genes, and their coding regions are identical, rearranged proto-myc cannot be sufficient for lymphomagenesis. By contrast, viral myc genes are oncogenic, owing to a 100-fold higher level of myc expression.

3. Primary Burkitt's lymphomas with normal chromosome 8, and with rearrangements of chromosome 8 that occur 3' from proto-myc and thus do not affect the structure and regulation of the proto-myc gene, indicate that proto-myc translocation is not necessary for Burkitt's lymphomas.

It follows