We have cloned a cDNA encoding a glycoprotein (S45) expressed in testis by screening a rat testis-specific cDNA library with the polyclonal rabbit antiserum to S45 and its nucleotide sequence was determined. The open reading frame is 1,071 base pairs long, which can encode a polypeptide of 356 amino acids. The cDNA was expressed in vitro by using rabbit reticulocyte lysate and a protein with molecular weight of 40 kDa, which agrees well to the predicted size of S45, was detected. Northern blot analysis showed that S45 mRNA reached maximum quantity at 21 days after birth.