Two of the genes whose mutations cause osteopetrosis are CLCN7 and OSTM1, encoding the chloride-proton (Cl−/H+) exchanger ClC-7 and its obligate β-subunit Ostm1 (osteopetrosis-associated transmembrane protein 1) (Chalhoub et al., 2003; Kornak et al., 2001; Lange et al., 2006; Leisle et al., 2011). The gene discussed is OSTM1; the disease is osteopetrosis.