SLC4A2 and osteopetrosis: AE2 is up-regulation during osteoclast differentiation, spermatogenesis, and enamel maturation3,19,20, and the dysfunction of AE2 is associated with achlorhydria, osteopetrosis, male infertile, and primary biliary cirrhosis3,6,19,21–23, showing the importance of AE2 in cellular pH homeostasis.