Partial loss of ATP6V1H function can lead to osteoporosis/osteopenia, and mutations in T-cell immune regulator 1 (TCIRG1), also known as V-ATPase V0 subunit A3, which is responsible for secreting acid, cause autosomal recessive osteopetrosis in humans (43–46). Here, TCIRG1 is linked to autosomal recessive osteopetrosis.