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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.

In order to understand cancer, it is necessary to identify cancer genes. The search for such genes and for mechanisms that generate such genes must take into consideration that at the cellular level cancer is a very rare event. The kind of cellular transformation that leads to cancer in vivo occurs only in about one out of 2 × 1017 mitoses in humans and animals. The basis for this estimate is that most animal and human cancers are derived from single transformed cells and are hence monoclonal [1–5], that humans and corresponding animals represent about 1016 mitoses (assuming 1014 cells that go through an average 102 mitoses), and that about one person in five dies from tumors [6].

A procedure has been developed to map the genetic elements of avian tumor virus RNA, which has a molecular weight of about 3 × 106 daltons and a poly(A) sequence at the 3’ end. For this purpose, about 30 RNase T1-resistant oli gonucleotides were ordered relative to the 3’-poly(A) terminus of the RNA, to construct an oligonucleotide map of viral RNAs. A cluster of seven envelope gene (env)-specific oligonucleotides, identified by their absence from the otherwise very similar oligonucleotide map of an envelope-defective deletion mutant (which lacks the major viral glycoprotein), mapped at a distance of 0.9 to 1.6 × 106 daltons from the poly(A) end of sarcoma virus RNA. A cluster of three sarcoma gene (src)-specific oligonucleotides, identified by their absence from the otherwise nearly identical oligonucleotide map of a transformation-defective deletion mutant mapped at a distance of 0.2 to 0.6 × 106 daltons from the poly(A) end of sarcoma virus RNA. The oligonucleotide maps of sarcoma viruses and of related deletion mutants were the same from the poly(A) end up to 0.2 × 106 daltons and included one terminal oligonucleotide, termed C, which is found in all avian tumor viruses tested so far. Preliminary mapping experiments ordering the src-specific and env-specific oligonucleotides of recombinants, selected for sarcoma and envelope genes of different parents, agree with those obtained by comparing maps of wild type viruses and deletion mutants. A partial genetic map consistent with these results suggests that the src gene maps between the env gene and the 3’-poly(A) end of viral RNA. This map reads: poly(A)-src-env-(pol, gag).

The main objective of cancer molecular biology is to identify cancer genes. Despite fierce efforts, this objective has still not been met [1–3]. As yet the only known cancer genes are the transforming onc genes of retroviruses. Typically these viruses initiate and maintain cancers with autonomous transforming genes that are dominant in susceptible cells [5]. The discovery of Single gene determinants of cancer in retroviruses has become a precedent that has infected cancer gene research. It has made retroviral onc genes the favorite models of cellular oncogenes, although the relevance of single-gene models to virus-negative tumors is as yet unknown. Fortunately, onc genes are either detrimental or at least useless to the viability of the virus and thus are not maintained by retroviruses. They are the produets of rare, genetic accidents, generated by illegitimate recombinations between retroviruses and cellular genes, termed proto-onc genes.

Avian acute leukemia viruses are a group of defective retroviruses which cause neoplasia in animals after short periods of latency. Transformation induced by the acute leukemia viruses usually manifest the direct expression of one or two viral onc genes that include characteristic elements transduced from cellular genes termed proto-oncogenes. Table 1 demonstrates the oncogenic properties of these viruses. Of particular interest to our laboratory is the MC29 subgroup of acute transforming viruses with onc-specific sequence termed myc as well as the avian myeloblastosis virus (AMV) subgroup with an onc-specific sequence termed myb. There are dramatic biologic differences between these two subgroups of viruses. As illustrated in Table 1, the MC29 subgroup possesses a broad spectrum of oncogenicity in vivo, causing leukemia and solid tumors in animals, and the ability to transform a variety of cells such as fibroblasts and hematopoietic cells in vitro. By contrast, the AMV subgroup has a much narrower spectrum of oncogenicity in vivo, causing leukemia, but not solid tumors, and possesses the ability to transform only hematopoietic cells in vitro.

The cellular precursor of Harvey and Balb sarcoma virus, termed proto-ras, from certain tumors registers most frequently as an apparent cancer gene in the gene transfer assay with NIH 3T3 cells [1–4]. The viral ras gene is a dominant transforming gene that elicits all features of oncogenic transformation in a single step upon transfer into established and primary human and animal fibroblast cells [5]. This encodes a single transforming protein of 21 kDA, p21 Ha-ras, which is presumed to be involved in the growth regulation of cells similar to G-proteins [6, 7]. However, the role of the cellular ras gene as a human cancer gene is debatable, as only 10% of the DNAs from certain cancer cells produce foci of transformed NIH 3T3 cells in the transfection assay. No consistent correlation between ras activation and any special type of human cancer is observed, thus 3T3-transforming proto-ras may not be necessary for malignancy [8, 9].

The interferon response may be induced by a variety of nonreplicating polyanionic macromolecules (Ho et al., 1967). Double-stranded RNA from a variety of sources is a potent inducer of interferon (Lampson et al., 1967; Tytell et al., 1967; Field et al., 1967a). The demonstration that double-stranded synthetic polyribonucleotides are also effective interferon inducers (Field et al., 1967b) stimulated the research presented in this paper.