Previously, we identified two genes, termed Inc-1 and Inb-1, that interact to enhance ecotropic murine leukemia virus induction in low virus strains of mice. Mice related to BALB/c in origin carry a locus termed Inc-1, whereas mice related to B6 carry an Inb-1 locus. Mice that carry both Inc-1 and Inb-1 yield 10- to 50-fold more virus-producing cells than parental strains on induction with halogenated pyrimidines in vitro and demonstrate enhanced murine leukemia virus production in vivo. Here, we show that mice related to BALB/c in origin, i.e., A, C3H/He, and SEC, have an Inc-1 locus that is allelic with that of BALB/c. The C57BR mouse strain has an Inb-1 locus that is allelic with that of B6, located on chromosome 8, 30 cM from Es-1. We also show that the Inc-1 locus of BALB/c mice is located on chromosome 5, 24 cM from Pgm-1 and 43 cM from Gus. Kozak and Rowe (6,8) and Ihle and co-workers (3) have shown that the ecotropic virus-inducing genes in BALB/c and B10 mice are located on chromosomes 5 and 8, respectively, with similar distances from the previously mentioned biochemical markers. Our data are consistent with two possibilities: Inc-1 and Inb-1 are part of the virus-inducing genes Cv-1 and Bv-1, respectively , or Inc-1 and Inb2-1 are tightly linked regulatory genes.

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