TCIRG1 and osteopetrosis: Disease mutations in subunits of the V0 domain result in, respectively, renal tubular acidosis with deafness (ATP6V0A4 (MIM 602722) (refs 3, 4, 5)), osteopetrosis (TCIRG1 or ATP6V0A3 (MIM 259700) (refs 3, 6, 7)) and cutis laxa with abnormal glycosylation (ATP6V0A2 (MIM 219200&278250) (ref. 8)).