Mouse contains two functional, but differentially expressed, cytochrome c genes. One of these genes is expressed in all somatic tissues so far examined. The other gene is expressed only in testis and is assumed to be spermatogenesis-specific. The nucleotide sequence of four mouse cytochrome c-like genes has been determined. One of these genes (MC1) contains an intron and encodes a polypeptide sequence identical to the published mouse somatic cytochrome c amino acid sequence. The other three genes can not properly encode a mouse cytochrome c protein and appear to be pseudogenes which have arisen via an insertion into the mouse genome of a cDNA copy of a cytochrome c mRNA molecule.