<p>The PKD (Polycystic Kidney Disease) domain was first identified in the Polycystic Kidney Disease protein, polycystin-1 (PDK1 gene), and contains an Ig-like fold consisting of a beta-sandwich of seven strands in two sheets with a Greek key topology, although some members have additional strands [<cite idref="PUB00001323"/>]. Polycystin-1 is a large cell-surface glycoprotein involved in adhesive protein-protein and protein-carbohydrate interactions; however it is not clear if the PKD domain mediates any of these interactions. </p><p>PKD domains are also found in other proteins, usually in the extracellular parts of proteins involved in interactions with other proteins. For example, domains with a PKD-type fold are found in archaeal surface layer proteins that protect the cell from extreme environments [<cite idref="PUB00014157"/>], and in the human VPS10 domain-containing receptor SorCS2 [<cite idref="PUB00035973"/>].</p>