Germ line DNA from all strains of mice contains numerous endogenous retroviruses. One of these viruses, a virus with xenotropic host range is induced from lymphocytes of most strains by treatment with B cell mitogens. Virus induction is amplified by 5-bromo-2'-deoxyuridine (BrdU) treatment. We report here studies of the genetic control of retrovirus induction from lymphocytes in crosses between BALB/cTif mice and noninducible 129/Rrj mice. We identify a novel locus, Bdv-1, which controls the expression of a reverse transcriptase-positive, defective retrovirus in BALB/cTif lymphocytes. In addition, we confirm previous reports that xenotropic virus is controlled by a locus, Bxv-1, mapping to chromosome 1. The two loci are nonlinked and respond differently to inducing stimuli. Bxv-1 is induced mainly by BrdU and only marginally by mitogen; in contrast, Bdv-1 is induced by mitogen and BrdU has little effect. The induction of these two loci is discussed with respect to B cell differentiation.

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