A murine cDNA encoding a homolog of the human multiubiquitin-chain-binding protein Mcb1 was isolated and sequenced from a mouse testis cDNA library. The encoded Mcb1 protein is highly conserved between mouse and human. Northern hybridisation showed expression of Mcb1 transcripts in all examined mouse tissues. In the testis, however, there is additionally a second, longer Mcb1 transcript in wild-type mice that is absent in the azoospermic W/Wv mutant mice, suggesting expression of this transcript in association with germ cell differentiation.