Mutations in the osteoclast vacuolar proton pump (TCIRG1) and the H+-Cl2 exchange transporter 7 (CLCN7) account for the majority of autosomal recessive osteopetrosis, whereas variants encoding the carbonic anhydrase II enzyme (CA2), a stabilizing b-subunit of the H+-Cl2 exchange transporter 7 (OSTM), and a lysosome-associated protein involved in vesicular trafficking (PLEKHM1) are less common [35,37]. The gene discussed is CA2; the disease is autosomal recessive osteopetrosis.