TCIRG1 and osteopetrosis: An advantage of targeting the V-ATPase a3 subunit to prevent osteolytic diseases is that osteoclast differentiation and fusion is unaffected by the absence of a3, as evidenced by the fact that, to date, all a3 mutations have resulted in osteoclast-rich osteopetrosis [96,131,132,133] (Figure 3, Table 2).