Previous studies have shown sequence similarity between a region of the autosomal dominant polycystic kidney disease (ADPKD) protein, polycystin-1 and a sea urchin sperm glycoprotein involved in fertilization, the receptor for egg jelly (suREJ). We have analysed sequence databases for novel genes encoding PKD/REJ-like proteins and found a significant region of homology to a large open reading frame in genomic sequence from human chromosome 22. Northern analysis showed that this is a functional gene [termed the polycystic kidney disease and receptor for egg jelly related gene ( PKDREJ )], but unlike polycystin-1, has a very restricted expression pattern; the approximately 8 kb transcript was found exclusively in testis, coincident with the timing of sperm maturation. The PKDREJ transcript was cloned by screening a testis cDNA library and RT-PCR which revealed a 7660 bp mRNA terminating with a 900 bp 3'UTR and a polyA tail. Comparison with genomic sequence showed that PKDREJ is intronless; sequencing the mouse orthologue revealed a similar structure. The predicted human PKDREJ protein has 2253 amino acids (calculated molecular mass 255 kDa) and sequence similarity over approximately 2000 amino acids with polycystin-1, corresponding to the predicted membrane associated region and the area of homology ( approximately 1000 amino acids) with the suREJ protein (the REJ module). The suREJ protein binds the glycoprotein coat of the egg (egg jelly), triggering the acrosome reaction, which transforms the sperm into a fusogenic cell. The sequence similarity and expression pattern suggests that PKDREJ is a mammalian equivalent of the suREJ protein and therefore may have a central role in human fertilization.