The defective transforming simian sarcoma virus (SSV) and its nondefective helper virus (SSAV) are retroviruses isolated from a fibrosarcoma of a pet woolly monkey. Together with the gibbon ape leukaemia viruses, they are the only group of retroviruses known to cause spontaneous and experimentally induced neoplasia in primates (see refs 1 and 2 for review). Molecular cloning has shown that SSV contains a 1.2-kilobase (kb) transformation-specific viral onc gene (v-sis)3,4 which, like other viral onc genes, is derived from a set of conserved cellular DNA sequences5. A human DNA fragment containing sequences homologous to the entire v-sis gene has also been cloned and analysed6. We present here experiments carried out on the cloned SSV genome which show that: (1) v-sis is distinct from other viral transforming (onc) genes; and (2) v-sis is derived from a woolly monkey naturally infected once with gibbon ape leukaemia virus (GaLV) and is to date the only onc gene of primate origin.
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