We describe two siblings with carpal and tarsal synostoses associated with transverse deficiencies of the toes. Mutation analysis of the NOG gene revealed a double missense mutation in both boys resulting in Pro42Ala and Pro50Arg. The parents were clinically unaffected, and these two mutations were not detected in their white blood cells or buccal mucosa. This indicates the presence of gonadal mosaicism or a low level of somatic mosaicism in one of the parents. © 2005 Wiley-Liss, Inc.