A cysteine-rich domain (CRD) is an essential component of a number of cell surface receptors, which are involved in multiple signal transduction pathways, particularly in modulating the activity of the Wnt proteins, which play a fundamental role in the early development of metazoans. CRD is also found in secreted frizzled related proteins (SFRPs), which lack the transmembrane segment found in the frizzled protein. The CRD domain is also present in the alpha-1 chain of mouse type XVIII collagen, in carboxypeptidase Z, several receptor tyrosine kinases, and the mosaic transmembrane serine protease corin. The CRD domain is well conserved in metazoans - 10 frizzled proteins have been identified in mammals, 4 in Drosophila and 3 in Caenorhabditis elegans. CRD domains have also been identified in multiple tandem copies in a Dictyostelium discoideum protein. Very little is known about the mechanism by which CRD domains interact with their ligands. The domain contains 10 conserved cysteines. The protein lin-17 is involved in cell type specification during Caenorhabditis elegans vulval development.