<p>The REJ (Receptor for Egg Jelly) domain is found in PKD1 <db_xref db="SWISSPROT" dbkey="P98161"/> and the sperm receptor for egg jelly <db_xref db="SWISSPROT" dbkey="Q26627"/>. The exact function of this domain is unknown. The domain is 600 amino acids long so is probably composed of multiple structural domains. There are six completely conserved cysteine residues that may form disulphide bridges. This region contains tandem PKD-like domains.</p><p>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) has been known for some time. 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 the predicted human PKDREJ protein is a mammalian equivalent of the suREJ protein and therefore may have a central role in human fertilization [<cite idref="PUB00005842"/>].</p>