The B9-C2 domain is found in proteins associated with the ciliary basal body. B9 domains were identified as a specific family of C2 domains. There are three sub-families represented by this family, notably, Mks1-Xbx7, Stumpy-Tza1 and Tza2 groups of proteins. Mutations in human Mks1 result in the developmental disorder Mechler-Gruber syndrome; mutations in mouse Stumpy lead to perinatal hydrocephalus and severe polycystic kidney disease. All the three distinct types of B9-C2 proteins cooperatively localize to the basal body or centrosome of cilia.