The MID1 gene in Xp22 codes for a novel member of proteins containing a RING finger, B-box, coiled-coil and a conserved C-terminal domain. Initially, three mutations in the C-terminal region were found in patients with Opitz G/BBB syndrome, a defect of midline development. Here we have determined the complete gene structure of the MID1 gene and have analyzed all nine exons for mutations in a set of 40 unrelated Opitz G/BBB patients. We now report six additional mutations all clustered in the carboxy-terminal domain of the MID1 protein. These data suggest that this conserved domain of the B-box proteins may play a fundamental role in the pathogenesis of Opitz syndrome and in morphogenetic events at the midline during blastogenesis.