We cloned and characterized the cDNA and the expression pattern of a novel member of the murine 'Disintegrin and Metalloprotease-Domain Family' (ADAM). The predicted protein sequence reveals highest homology to the testase-subgroup, composed of ADAM 24, ADAM 25 and ADAM 26 and is therefore called testase 4. Reverse transcription-polymerase chain reaction showed a strong expression of testase 4 in the adult testis, but not in any other organ or embryonic stage tested. Careful characterization by in situ hybridization confirmed specific expression of testase 4 in maturing sperm cells of 6-week-old mice, whereas no specific hybridization pattern was detectable in testes of 2.5-week-old mice. These data indicate a correlation between testase 4 expression and spermatogenesis and/or fertilization.