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 [1]. 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 [2]; mutations in mouse Stumpy lead to perinatal hydrocephalus and severe polycystic kidney disease [3]. All the three distinct types of B9-C2 proteins cooperatively localise to the basal body or centrosome of cilia.

1: Identification of novel families and classification of the C2 domain superfamily elucidate the origin and evolution of membrane targeting activities in eukaryotes. Zhang D, Aravind L; Gene. 2010;469:18-30. PMID:20713135

2: MKS1, encoding a component of the flagellar apparatus basal body proteome, is mutated in Meckel syndrome. Kyttala M, Tallila J, Salonen R, Kopra O, Kohlschmidt N, Paavola-Sakki P, Peltonen L, Kestila M; Nat Genet. 2006;38:155-157. PMID:16415886

3: The stumpy gene is required for mammalian ciliogenesis. Town T, Breunig JJ, Sarkisian MR, Spilianakis C, Ayoub AE, Liu X, Ferrandino AF, Gallagher AR, Li MO, Rakic P, Flavell RA; Proc Natl Acad Sci U S A. 2008;105:2853-2858. PMID:18287022