ACP5 and osteopetrosis: NFATc1 is essential for the expression of osteoclastogenic genes encoding tartrate-resistant acid phosphatase (TRAP), cathepsin K, matrix metalloproteinase 9, and beta 3-integrins.4–6 Aberrant regulation of osteoclast formation can contribute to the pathogenesis of bone diseases such as osteoporosis, osteopetrosis, and Paget’s disease of bone.7