We previously reported our characterizations of the B-lymphoma tumors induced in a highly susceptible line of chickens (15I5 x 72) by the avian leukosis virus RAV-1 (Proc. Natl. Acad. Sci. 78:3418-3422, 1981). We demonstrated that in >90% of the tumors, the RAV-1 provirus is integrated near a cellular oncogene, c-myc. In the present study, we devised a simple approach, relying on SalI digestion, for further defining the locations and orientations of the proviruses with respect to the c-myc gene. We report here that in the great majority of cases the provirus is situated upstream from and in the same transcriptional direction as the c-myc gene - a configuration compatible with the promoter-insertion model proposed by Hayward et al. (Nature [London] 290:475-480, 1981).
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