The genomes of inbred strains of mice harbour multiple integrated endogenous retroviruses and many of these sequences are homologous to murine leukaemia virus (MLV). Gv1 is a single locus that co-ordinately regulates the expression of proviral sequences, although its mode of action is unknown. Cloning and characterisation of this gene may therefore establish how these endogenous sequences are controlled. A reliable quantitative nuclease protection assay has been established to type mice for their Gv1 genotype. Use of this assay demonstrated that the segregation of a single gene is responsible for the proviral expression phenotypes observed, and that not all classes of provirus are responsive to Gv1. Genome scanning studies using a series of microsatellite probes on pooled DNA samples allowed linkage to be excluded from 74% of the genome, but did not reveal a map position for Gv1. Results from REVEAL-PCR identified two markers that showed linkage to the Gv1 locus. Both were mapped by somatic cell and radiation hybrid panels to chromosome 13. Further microsatellite markers were analysed around the region of interest and a detailed genetic map was established from a panel of over 1000 backcross animals, placing Gv1 in a 0.27 cM region approximately 37 cM from the centromeric end of the chromosome. Cloning the gene by a positional approach can now be considered. As polytropic and modified polytropic MLVs differ in their responsiveness to Gv1, LTR sequences polymorphic between the two classes may therefore be a target for the gene. Preliminary data from both gel-mobility assays and DNA footprinting show that there appear to be polytropic-specific binding sites in the LTR U3 region.