145SC5 is a Y chromosomal repetitive sequence isolated from a BALB/c mouse and is unique in that it detects poly(A)-containing transcripts in the testis. We determined nucleotide sequences of 145SC5 and a related cDNA clone designated PC11 and compared their sequences to that of pYMT2/B, another related cDNA clone. 145SC5 and PC11 are almost identical (97%), while PC11 and pYMT2/B share 84% identity. In situ hybridization and Southern blot analysis with XXSxr male mice demonstrated that 145SC5-related sequences were distributed over the entire length of the Y chromosome including the short arm. Two types of Y chromosome are present in inbred mouse strains: the Mus musculus musculus type and M.m. domesticus type. Among 26 inbred strains surveyed, 145SC5 detected polymorphims only in the M.m. domesticus Y chromosome.