Acute Leukemia Virus Research Articles

Fusion Proteins in Retroviral Transformation

Publisher Summary Retroviral transforming proteins are coded for by oncogenes. The mechanism of the uptake of oncogenes by retroviruses often results in fusion with replicative genes. Fusion of oncogenes to retroviral replicative genes generates protein products that are also fused. These proteins contain viral and oncogene information in a single polypeptide chain. The viral structural portion provides a convenient antigenic marker that allows detection of such oncogene products with antiviral antibodies, which are easily available. In contrast, oncogene products are generally poorly antigenic because they closely resemble cellular proteins. Therefore, oncogene products without viral structural portions are much more difficult to detect. This chapter discusses several known retroviral fusion proteins and the potential role of the viral structural protein portions in the transforming proteins. It summarizes the oncogenes, their viral origin, their gene products, and some of their characteristic properties. The avian retroviruses comprise four groups; three consist of the acute avian leukemia viruses, such as the myelocytomatosis virus family (MC29), the avian erythroblastosis viruses (AEV), and the avian myeloblastosis viruses (AMV). The fourth group is represented by the avian sarcoma viruses.

The mil/raf and myc oncogenes: molecular cloning and in vitro mutagenesis.

Avian retrovirus MH2 is a member of the MC29 subgroup of avian acute leukemia viruses which includes the four independently isolated viruses MC29, CMII, OKIO, and MH2 [4]. The genetic hallmark of these viruses is the presence in their genomes of unique transformation-specific sequences, termed v-myc [4, 5], which are of cellular origin [3–5, 13]. In contrast to MC29, CMII, and OKIO which induce predominantly leukemic diseases, MH2 induces predominantly liver and kidney carcinomas [1, 2].

Characterization of the number of carbohydrate chains on the avian erythroblastosis virus erb B gene product and their role in transformation.

Avian erythroblastosis virus, AEV, is a replication-defective avian acute leukemia virus that causes acute erythroid leukemia and sarcomas on injection into young chicks. Analysis of the genome of AEV revealed that it was 5.0 kilobases in length and that the gene order was 5′-gag-erb-envpoly A-3′ where gag and env represent partially deleted structural genes and erb is a 3.25 kilobases sequence of nonviral origin [2, 13]. Subsequently, sequences homologous to erb could be found in cellular DNA from all vertebrate species tested [16, 19]. This evidence suggested that the erb sequences represented the viral oncogene of AEV, and led to studies attempting to define the products of this gene.

Transformation of erythroid cells by rous sarcoma virus (RSV)

RSV transforms several nonhematopoietic cell types and as reported here also has the capacity to transform hematopoietic cells of the erythroid lineage. In vitro, the three RSV isolates tested induced erythroblast-like colonies in infected bone marrow cells that were distinguishable by size and cell arrangement from those induced by avian erythroblastosis virus (AEV). Also in contrast to AEV-transformed erythroblast cultures, isolated cell colonies induced by RSV required complex growth conditions in liquid medium similar to the in vitro conditions necessary for erythroblasts transformed by the acute leukemia virus E26. Temperature-shift experiments using temperature-sensitive ( ts) NY68 RSV revealed that when grown at the nonpermissive temperature (42°), mutant-infected cells became benzidine positive and partially differentiated into erythrocytes. Wild-type ( wt) RSV-transformed cells did not undergo similar changes. However, both wt RSV-, and to a greater extent, ts RSV-transformed cultures at the permissive temperature (37°) did contain populations of spontaneously differentiating erythroid cells signifying that the transforming activity of the virus did not fully arrest erythroid maturation. In addition, the RSV-transformed cells did express tyrosine kinase activity. When injected intravenously into birds, RSV induced an erythroblastosis-like disease similar to AEV but also caused fibrosarcomas and leg paralysis. These results show that RSV can alter the pattern of erythroid differentiation in a manner similar to, but distinct from, AEV and indicate that the tyrosine-specific pp60 src kinase is involved in erythroid cell transformation. Since the arc and erb B proteins share a significant amino acid homology, these data suggest that both may also share a common functional homology.

Ts mutants of E26 leukemia virus allow transformed myeloblasts, but not erythroblasts or fibroblasts to differentiate at the nonpermissive temperature

The myb,ets-containing avian acute leukemia virus E26 transforms myeloblasts, erythroblasts, and fibroblasts in culture and causes a mixed erythroid/ myeloid leukemia in chicks. We report the isolation and characterization of four E26 mutants that are temperature-sensitive ( ts) for myeloblast transformation. At the permissive temperature, tsE26-transformed myeloid cells resemble macrophage precursors and proliferate rapidly, provided the growth medium contains chicken myelomonocytic growth factor (cMGF). When shifted to the nonpermissive temperature the cells stop growing and differentiate into macrophage-like cells, as determined by their expression of morphological, functional, and antigenic markers of normal macrophages. They also lose their responsiveness to cMGF and secrete a cMGF-like factor. Ts mutants of E26 retain their leukemogenicity and their ability to transform both erythroblasts and fibroblasts at the nonpermissive temperature, suggesting that the myb oncogene of E26 causes myeloblast transformation and that ets is responsible for erythroblast and fibroblast transformation.

Progressive Cutaneous Herpes Simplex Infection in Acute Myeloblastic Leukemia

A patient had acute myeloblastic leukemia and extensive progressive cutaneous herpes simplex virus infection. Complete and rapid healing of skin lesions with remission of the leukemia occurred during 15 days of therapy with human leukocyte interferon and minimal doses of cytarabine hydrochloride. Pharmacokinetic studies showed that the patient had a defective antiviral interferon response, which was effectively corrected by treatment with interferon alpha. This may help to explain the dramatic response of both conditions to therapy.

Identification of a 36,000-molecular weight, gag-related phosphoprotein in lymphoma cells transformed by radiation leukemia virus.

Radiation leukemia virus (RadLV) causes thymic lymphoma in 90% of susceptible mice after a latent period of several months. The virally encoded polypeptides produced by RadLV-induced lymphoma cells were analyzed by immunoprecipitation and NaDodSO4/polyacrylamide gel electrophoresis. Along with the expected precursor and mature forms of gag and env gene products, a polypeptide of 36,000 molecular weight (p36) was precipitated by anti-gag antisera. It was not precipitable by normal sera or anti-env antibodies. Like the gag-associated fusion proteins of some acute leukemia viruses, p36 was found to be phosphorylated in vivo, although it lacked detectable ATP-specific protein kinase activity in vitro. By kinetics during pulse-chase labeling experiments and by comparison of two-dimensional tryptic peptide maps, this protein is not an intermediate in gag precursor processing. One lymphoma cell line is described that resembles a nonproducer RadLV-transformant, synthesizing relatively large amounts of p36 in the absence of Pr66gag or p30 production. Several RadLV-induced lymphoma cell lines also produce p36, while it was not detectable in the radiation-induced lines tested. In addition, p36 was not produced by mouse or mink fibroblasts or cultured thymocyte cell lines infected with virus passaged from the RadLV-induced lymphomas. We conclude that p36 may represent a previously unrecognized transformation-related protein induced directly or indirectly by infection with RadLV.

Avian myeloblastosis virus and E26 virus oncogene products are nuclear proteins.

The defective acute leukemia viruses avian myeloblastosis virus (AMV) and E26 virus each contain an inserted cellular sequence related to the same highly conserved cellular gene, proto-amv. The oncogenes of these two retroviruses differ from this cellular proto-oncogene in gene structure, transcript structure, and gene product. The product of the AMV oncogene (myb) is a 48,000 Mr protein, p48myb, encoded by a transduced segment (amv) of proto-amv flanked by short helper-virus-derived terminal sequences. The E26 virus oncogene product is a 135,000 Mr protein, p135gag-amve-ets, encoded by significant portions of a viral structural gene (gag), sequences related to proto-amv (amve), and additional E26-specific sequences (ets) transduced from cellular proto-ets. Both p48myb and p135gag-amve-ets transforming proteins are located in the nucleus of cells transformed by these viruses. A protein of 110,000 Mr which is specifically immunoprecipitated by antisera to amv peptides and may be the product of the normal cellular gene (proto-amv) has been located in the cytoplasm of cells that express proto-amv mRNA.

OKIO-Transformed Cell Lines: Viral Component Generation and Tumorigenicity

Stable nonproducer (NP) cell lines transformed by the avian acute leukemia virus OK10 were established. All 13 studied NP quail cell lines released into the culture medium noninfectious, mostly reverse-transcriptase-negative particles containing the usual gag proteins. Infectious, transforming OK10 virus pseudotypes could be recovered by rescue with helper virus. p200, the putative transforming protein of OK10, was identified in in vitro translates of RNA from the noninfectious particles and in immunoprecipitates of cell extracts of the NP clones analyzed. Two NP clones, the reverse-transcriptase-negative B5 and -positive 9C cell lines, displayed striking differences in in vivo tumorigenicity. Although B5 induced no tumors in quails, 9C caused multiple tumors and cells derived from several tumors could be passaged in vitro. At no time during the in vivo passage or during more than 1 year of in vitro culture have the particles released by 9C cells acquired infectious properties. Possible reasons for the observed differences in tumorigenicity between the OK10 virus-transformed 9C and B5 NP cell lines are discussed.

Quail embryo fibroblasts transformed by four v-myc-containing virus isolates show enhanced proliferation but are non tumorigenic.

Quail embryo fibroblasts infected with any of the four natural avian myc gene-containing virus strains (MC29, CMII, OK10 and MH2) or with the myb, ets-containing E26 acute leukemia virus, were examined for their expression of several transformation-associated parameters. All myc-containing viruses, but not E26 or Rous sarcoma virus (used as a control) induced a dramatic stimulation of cell proliferation. In addition, the myc virus-transformed cells exhibited prominent nucleoli, possibly as a consequence of their increased proliferation. Cells transformed by MC29, OK10, MH2 and E26 were capable of growing in semi-solid medium and showed a loss of actin cables and, in most cases, of an ordered fibronectin distribution. All of the myc virus-transformed fibroblasts, as well as the E26-transformed cells, were unable to form tumors in nude mice, indicating that the myc gene (and the myb/ets genes) are not sufficient for the induction of a fully malignant phenotype in avian fibroblasts.

Two unrelated cell-derived sequences in the genome of avian leukemia and carcinoma inducing retrovirus MH2.

Molecularly cloned proviral DNA of avian replication-defective retrovirus Mill Hill No. 2 (MH2) was analyzed. The MH2 provirus measures 5.5 kb including two long terminal repeats (LTR), and contains a partial complement of the structural gene gag, 1.5 kb in size, near the 5' terminus, and a 1.3-kb segment of the v-myc transforming gene near the 3' terminus. These v-myc sequences are closely related to the v-myc transforming gene of avian acute leukemia virus MC29, and to the cellular chicken gene c-myc. The gag and myc domains on the MH2 provirus are separated by unique sequences, 1.3 kb in size and termed v-mil, which are unrelated to v-myc, or to other oncogenes or structural genes of the avian leukemia-sarcoma group of retroviruses. Normal chicken DNA contains sequences closely related to v-mil, termed c-mil. Analyses of chicken c-mil clones isolated from a recombinant DNA library of the chicken genome reveal that c-mil is a single genetic locus with a complex split gene structure. In the MH2 genome, v-mil is expressed via genome-sized mRNA as a gag-related hybrid protein, p100gag-mil, while v-myc is apparently expressed via subgenomic mRNA independently from major coding regions of structural genes. The presence in the MH2 genome of two unrelated cell-derived sequences and their independent expression may be significant for the oncogenic specificities of this virus.

Avian oncovirus MH2: molecular cloning of proviral DNA and structural analysis of viral RNA and protein.

Viral RNA, molecularly cloned proviral DNA, and virus-specific protein of avian retrovirus MH2 were analyzed. The complexity and sequence conservation of the transformation-specific v-myc sequences of MH2 RNA were compared with those of the other members of the MC29 subgroup of acute leukemia viruses, MC29, CMII, and OK10, and with chicken cellular c-myc sequences. All T1 oligonucleotides mapping within the 1.3-kilobase coding region of MC29 v-myc have homologous counterparts in the RNAs of all MC29 subgroup viruses and in c-myc. These counterparts are either identical in composition or altered by single point mutations. Hence, the 47,000-dalton carboxy-terminal sequences of the transforming proteins of these viruses and of the cellular gene product are probably highly conserved but may contain single amino acid substitutions. T1 oligonucleotide mapping of MH2 RNA indicated that the MH2 v-myc sequences map close to the 3' end of viral RNA. A genomic library of an MH2-transformed quail cell line was prepared by using the Charon 4A vector system. By screening with an myc-specific probe, a clone containing the entire MH2 provirus (lambda MH2-1) was isolated. Digestion of cloned DNA with KpnI yielded a 5.1-kilobase fragment hybridizing to both gag- and myc-specific probes. Further restriction mapping of lambda MH2-1 DNA showed that about 1.6 kilobases of the gag gene are present near the 5' end of proviral DNA, and the conserved part of v-myc, i.e., 1.3 kilobases, is present near the 3' end of proviral DNA. These two domains are separated by a segment of at least 1 kilobase of different genetic origin, including additional unique sequences unrelated to virion genes. Tryptic peptide analysis of the gag-related protein of MH2, p100, revealed gag-specific peptides and several unique methionine-containing peptides. One of the latter is possibly shared with the polymerase precursor protein Pr180gag-pol, but no myc-specific peptides, defined for the MC29 protein p110gag-myc, appear to be present in MH2 p100. The data on viral RNA, proviral DNA, and protein of MH2 reveal a unique genetic structure for this virus of the MC29 subgroup and suggest that its v-myc gene is not expressed as a gag-related protein.

Proteins encoded by v-myc and c-myc oncogenes: Identification and localization in acute leukemia virus transformants and bursal lymphoma cell lines

We have prepared an antiserum against a synthetic dodecapeptide whose sequence corresponds to the C terminus of the MC29 v- myc protein. This antiserum (anti-v- myc 12C) specifically precipitates the known gag-myc fusion proteins produced by the defective leukemia viruses MC29, CMII, and OK10, but does not react with gag-precursor or product proteins. In addition, proteins of 62 kd and 61 63 kd are precipitated by anti-v- myc 12C from OK10 and MH2 transformants, respectively. The serum also recognizes comigrating 62 kd proteins from three chicken bursal lymphoma cell lines and from the products of in vitro translation of c- myc-specific mRNA. All of these myc-related proteins are phosphorylated and all appear to be localized in the cell nucleus. In uninfected quail cells, anti-v- myc 12C also recognizes a candidate c- myc protein of 60 kd, which does not appear to be phosphorylated and is present in low levels relative to v- myc and lymphoma c- myc proteins.

Decreased DNA-binding ability of purified transformation-specific proteins from deletion mutants of the acute avian leukemia virus MC29.

Avian myelocytomatosis virus MC29 is a highly oncogenic replication-defective retrovirus that predominantly affects hematopoietic cells and causes acute leukemia in vivo and that transforms hematopoietic cells as well as fibroblasts in vitro. The transformation-specific sequence, v-myc, is expressed as part of a fusion protein that contains the viral structural protein p19. By use of monoclonal antibodies against p19 we showed that the v-myc-encoded protein is located in the nucleus of MC29-transformed fibroblasts and that after purification over an immunoaffinity column the protein binds to double-stranded DNA. In this report we describe the analysis of the v-myc gene product from MC29-transformed bone marrow cells. The immunoaffinity column-purified protein from these cells also bound to DNA and was indistinguishable from the purified protein from MC29-transformed fibroblasts. In addition, the v-myc gene products from fibroblasts transformed by three nonconditional mutants of MC29--which transform hematopoietic cells with a markedly decreased efficiency in vivo and in vitro but still transform fibroblasts in vitro, expressing deleted v-myc proteins--were analyzed. In contrast to the wild-type protein, the purified mutant proteins had decreased DNA-binding abilities. Furthermore, a preferential binding of the wild-type protein to poly(dG) . poly(dC) duplexes was observed. Such a specificity was lost with a mutant protein. These results provide evidence that the interaction of the v-myc protein with DNA may be directly involved in transformation of the hematopoietic target cells. Further, the transformation-specific fusion proteins purified from cells transformed by avian erythroblastosis virus, which belongs to a different class of acute leukemia viruses, and by Fujinami sarcoma virus were found not to be DNA-binding proteins, suggesting the existence of different transformation mechanisms.

Genome structure of HBI, a variant of acute leukemia virus MC29 with unique oncogenic properties.

We have analyzed the viral RNA of a variant of avian acute leukemia virus MC29, termed HBI. This virus was isolated during in vitro passage of a partially transformation-defective (td) mutant of MC29 (td10H-MC29) in chicken macrophages. While td10H-MC29 has a reduced ability to transform macrophages in vitro or to induce tumors in vivo, HBI-MC29 transforms macrophages efficiently and induces in vivo a high incidence of lymphoid tumors. Electrophoretic analysis of HBI-MC29 genomic RNA revealed that it has a complexity of 5.7 kilobases, like the RNA of wild-type (wt) MC29, and that it is 0.6 kilobases longer than the 5.1-kilobase RNA of the deletion mutant td10H-MC29. Analysis of the viral RNAs of two clonal isolates of HBI-MC29 by T1 oligonucleotide fingerprinting showed that sequences from the viral transformation-specific region, v-myc, which are deleted in td10H RNA, are present in HBI RNA. Moreover, hybridization of HBI RNA to molecularly cloned subgenomic fragments of wtMC29 proviral DNA, followed by fingerprint analysis of hybridized RNA, showed that the entire v-myc-specific RNA sequences defined previously are present. Hybridization to cloned DNA of the normal chicken locus c-myc shows a close relationship between HBI v-myc RNA and c-myc DNA, especially in the sequences which were deleted from td10H-MC29. T1 oligonucleotide maps of HBI and td10H RNAs were prepared and compared. Total conservation of the oligonucleotide pattern is observed in the overlapping v-myc regions, while the partial structural genes gag and env show some variations, most of which can be directly proven to be due to point mutations or recombination with helper viral RNAs that were analyzed in parallel. Recombination of td10H-MC29 with c-myc, followed by recombinational and mutational changes in the structural genes during passage with helper virus, could be a possible explanation for the origin of HBI.

Avian acute leukemia virus OK 10: analysis of its myc oncogene by molecular cloning.

Several DNAs representing the genome of the avian acute leukemia virus OK 10 were isolated by molecular cloning from a transformed quail cell line, 9C, which contained at least six OK 10 proviruses. Recombinant lambda phages harboring the OK 10 genome and additional flanking cellular DNA sequences were studied by restriction endonuclease mapping and hybridization to viral cDNA probes. Six of the clones represented complete proviruses with similar, if not identical, viral sequences integrated at different positions in the host DNA. The organization of the OK 10 genome was determined by electron-microscopic analysis of heteroduplexes formed between the cloned OK 10 DNA and DNAs representing the c-myc gene and the genomes of two other avian retroviruses, Rous-associated virus-1 and MC29. The results indicated that the OK 10 proviral DNA is about 7.5 kilobases in size with the following structure: 5'-LTR-gag-delta polmyc-delta env-LTR-3', where LTR indicates a long terminal repeat. The oncogene of OK 10, v-mycOK 10, forms a continuous DNA segment of around 1.7 kilobases between pol and env. It is similar in structure and length to the v-myc gene of MC29, as demonstrated by restriction endonuclease and heteroduplex analyses. Two of the OK 10 proviruses were tested in transfection experiments: both DNAs gave rise to virus with the transforming capacities of OK 10 when Rous-associated virus-1 was used to provide helper virus functions.

Specificity in the immunosuppression induced by avian reticuloendotheliosis virus.

Several parameters of the cellular and humoral immune responses of chickens infected with reticuloendotheliosis virus (REV-T), an avian defective acute leukemia virus, or with its helper virus, reticuloendotheliosis-associated virus (REV-A), were evaluated. Spleen cells from chickens infected with REV-T (REV-A) or REV-A exhibited depressed mixed lymphocyte and mitogen responses in vitro. Allograft rejection was delayed by 6 to 14 days in birds infected with REV-A. The specific antitumor cell immune response was also studied by a 51Cr-release cytotoxicity assay. Lymphocytes from chickens infected with low numbers of the REV-T-transformed cells exhibited significant levels of cytolytic reactivity against the 51Cr-labeled REV-T tumor cells in vitro. The mitogen response of lymphocytes from these injected birds was similar to that of uninjected chickens. In contrast, lymphocytes from chickens injected with higher numbers of REV-T-transformed cells exhibited suppressed mitogen reactivity and failed to develop detectable levels of cytotoxic activity directed against the REV-T tumor cells. These results suggest that the general depression of cellular immune competence which occurs during REV-T (REV-A) infection could contribute to the development of this acute leukemia by inhibiting the proliferation of cytotoxic cells directed against the tumor cell antigens. The cytotoxic effect observed after the injection of chickens with non-immunosuppressive levels of REV-T-transformed cells appears to be specific for the REV-T tumor cell antigens since cells transformed by Marek's disease virus or avian erythroblastosis virus were not lysed. In marked contrast, birds whose cellular immune responses were suppressed by infection with REV-A were capable of producing a humoral immune response to viral antigens. Detectable levels of viral antibody, however, did not appear until 12 to 15 days after REV-A infection. Since REV-T (REV-A) induced an acute leukemia resulting in death within 7 to 14 days, it appears unlikely that the ability of chickens to make antiviral antibody influences the development of lethal reticuloendotheliosis.

Detection of avian hematopoietic cell surface antigens with monoclonal antibodies to myeloid cells: Their distribution on normal and leukemic cells of various lineages

The isolation and characterization of monoclonal antibodies reacting with cell surface antigenic determinants of normal and leukemic avian hematopoietic cells is described. The antibodies were produced by immunizing mice with normal macrophages, as well as with myeloid cells transformed with the avian acute leukemia viruses MC29, AMV and E26. Eleven antibodies were characterized for their reactivity with a variety of normal and leukemic cells of the myeloid, B- and T-lymphoid and of the erythroid cell lineage. Using several methods, they could be subdivided into five distinct types: 1. I. Four antibodies were specific for the myeloid lineage, predominantly reacting with immature myeloid cells. 2. II. One antibody reacted with mature and immature myeloid cells as well as with T-lymphoid cells. 3. III. Four antibodies reacted with myeloid, erythroid and T-lymphoid cells. 4. IV. One antibody reacted with myeloid as well as with T- and B-lymphoid cells. 5. V. One antibody reacted with all kinds of chicken hematopoietic cells except erythrocytes. The first type of antibodies detected glycoproteins with MWs of 170 and 130 kD. The pattern of antigens precipitated varied with the different monoclonal antibodies of this group. The antibody of the fourth type precipitated a 30 kD polypeptide from extracts of myeloid and lymphoid cells. None of the other antibodies precipitated any detectable proteins.

Avian acute leukemia viruses.

On the basis of their oncogenic properties, avian retroviruses can be divided into three groups: (a) sarcoma viruses (ASV), which cause solid tumors of the connective tissue after a short latency period; (b) lymphatic leukemia viruses (LLV), which cause a B-cell lymphoma only after a long latency period of approximately 6 months or more; and (c) acute leukemia viruses (ALV), which upon infection of young chickens rapidly bring about the death of the birds from a variety of different neoplasms, usually, but not exclusively, of the hematopoietic system (Purchase and Burmester 1973; Hanafusa 1977; Graf and Beug 1978). This classification is not absolute, in that there are overlaps in the oncogenic spectra of these viruses. For example, certain LLV have been shown to cause erythroblastosis and occasionally sarcomas (Purchase and Burmester 1973); similarly, certain ALV can also cause sarcomas (Rothe-Meyer and Engelbreth Holm 1933; Graf et al. 1977). However, by using this classification the seven field isolates of avian retroviruses listed in Table 1 have been grouped together as acute leukemia viruses. There is another isolate, reticuloendothelosis virus (REV-T), which is, by this definition, an ALV; however, since it is not related to the classical avian leukosis/sarcoma complex as judged by nucleic acid homology, it will not be included here. Interested readers are referred to Witter (1978) and Graf and Stehelin (1982). There are numerous reports in the literature regarding the isolation of other ALV; unfortunately, these strains are no longer available (see Graf and Beug 1978). Consequently, it is only the viruses listed in Table 1 which will be considered in this review.KeywordsErythroid CellAvian Myeloblastosis VirusAvian Leukosis VirusAvian Sarcoma VirusImmature MacrophageThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Viral oncogenes and cellular prototypes.

The structural hallmark of retroviral transforming onc genes is a specific RNA sequence that is unrelated to the essential retroviral genes but closely related to certain cellular prototypes termed proto-onc genes. Two types of onc genes have been distinguished. Type I are onc genes which utilize elements of specific sequences only to encode a transforming protein. Type II onc genes are hybrids which utilize essential viral (typically gag) and specific RNA sequences to encode transforming proteins. Comparisons between viral onc genes and cellular proto-onc genes are reviewed in the light of two competing models for proto-onc function: the quantitative model, which holds that viral onc genes and cellular proto-onc genes are functionally the same and that transformation is the result of enhanced dosage of a cellular proto-onc gene; and the qualitative model, which holds that they are different. Structural comparisons between viral onc genes and cellular prototypes have demonstrated extensive sequence homologies in the primary structures of the specific sequences. However, qualitative differences exist in the structure and organization of viral onc genes and cellular prototypes. These include differences in promoters, minor differences in the primary structure of shared sequences, and absolute differences such as in the presence of sequences which are unique to viral onc genes or to corresponding cellular genetic units. For example, type II hybrid onc genes of retroviruses share only their specific but not their gag-related elements with the cell, and cellular proto-onc genes are interrupted by sequences of nonhomology relative to viral onc genes. In addition, proto-onc gene units may include unique cellular coding sequences not shared with viral onc genes. There is circumstantial evidence that some proto-onc genes are potentially oncogenic after activation (quantitative model) or modification (qualitative model). Activated by an adjacently integrated retroviral promotor, the cellular prototype of the onc gene of the avian acute leukemia virus MC29 was proposed to cause lymphoma and activated by ligation with viral promoter sequences two proto-onc DNAs, those of Moloney and Harvey sarcoma viruses, were found to transform mouse 3T3 cell lines. Mutations presumably conferred 3T3 cell-transforming ability to the proto-onc gene of Harvey sarcoma virus that has been isolated from a human bladder carcinoma cell line. In no case has an unaltered proto-onc as yet been shown to be necessary and sufficient for carcinogenesis. Despite this and structural differences between viral onc genes and cellular proto-onc genes, we cannot at present conclusively distinguish between the quantitative and the qualitative models because a genetic and functional definition of most viral onc genes and of all cellular prototypes of viral onc genes are not as yet available.