Wnt9b is a member of the WNT family and is a ligand for members of the frizzled family of seven transmembrane receptors. The WNT gene family consists of structurally related genes that encode secreted signaling proteins. Wnt9b is expressed in the inductive epithelia and is essential for the development of mesonephric and metanephric tubules and caudal extension of the Müllerian duct. Wnt9b plays a central role in the regulation of mesenchymal to epithelial transitions underlying organogenesis of the mammalian urogenital system. The gene is clustered with WNT3, another family member, in the chromosome 17q21 region.