Genetic analysis of radiation-induced deletion mutations involving the chromosome 7 albino (c) locus has expanded the functional map of this 6 to 11-cM region of the mouse genome. To generate one of many points of molecular access necessary for intensifying the analysis of the genes and phenotypes associated with this particular complex of deletions, we have cloned an endogenous ecotropic leukemia provirus (Emv-23), known to be closely linked to c, along with its flanking chromosome 7 sequences. A unique-sequence probe (23.3), derived from a region immediately 5' to the proviral integration site, was found to map less than 0.5 cM from c in a standard backcross analysis. Southern blot analysis of DNAs from animals carrying homozygous or overlapping albino deletions demonstrated that the 23.3 probe was deleted in several relatively small c-region deletions. The deletion mapping of the 23.3 probe places the Emv-23 locus between c and Mod-2, just proximal to a region important for male fertility and juvenile fitness. Mapping of this locus also provides a refinement of the genetic/deletion map for several mutations within this deletion complex.