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This is a chicken virus unrelated to the human pathogen respiratory syncytial virus, also abbreviated RSV.

levels. This high expression level contributes to the dominant mode of action of retroviral oncogenes. Of course, the replacement of pol or other viral genes by an oncogene sequence obliterates the ability of the virus to replicate autonomously, rendering it 'defective'. For replication and propagation, defective retroviruses need intact replication-competent helper viruses that supply the proteins required for reverse transcription, integration, packaging and maturation. This requirement may explain why acutely transforming retroviruses are (fortunately) rare. Conversely, the unique ability of RSV to replicate autonomously may have contributed to its early isolation already one century ago.

Figure 4.1 Prototypic retrovirus and onco-retrovirus genomes ALV (avian leukosis virus), RSV (Rous sarcoma virus) and MLV (murine leukemia virus)

Meanwhile, more than two dozen oncogenes have been isolated from retroviruses and have been biochemically characterized. For a selection of these, their origin, cellular localization and main biochemical function are listed in Table 4.1. Several points can be noted in this compilation. (1) Some oncogenes appear to be similar. In some cases this is due only to the nomenclature: v-myb and v-myc are not overtly similar beyond being transcription factors, but have both been discovered in retroviruses causing myeloid leukemias. In contrast, Ki-ras and Ha-ras are indeed highly similar proteins. This points to the existence of oncogene families. (2) The products of retroviral oncogenes appear to cover a relatively limited range of biochemical functions. Protein kinases like v-erbB, v-src and v-raf and transcriptional activators like v-fos, v-jun, v-myb and v-myc comprise the majority. Others are receptors, like v-erbB or v-fms, or proteins transducing signals from receptors, like the ras proteins. A limited number of other functions constitute the remainder. (3) Some oncogenic retroviruses carry two oncogenes which cooperate

Table 4.1. Some important retroviral oncogenes

Oncogene

Virus

Species

Tumor type

Localization in the

Main biochemical

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