Using mouse probes specific to cytochrome cs and to cytochrome cT, the single-copy genes encoding these two proteins have been mapped to paralogous chromosomal regions by analysis of restriction fragment length variants in interspecific crosses. The gene for cytochrome cs, Cycs, maps to a position between Tcrb and Cbl-1 on proximal mouse Chromosome 6, and the gene for cytochrome cT, Cyct, maps between Gad-1 and Sfpi-1 on mouse Chromosome 2.