AFF has been reported in hypophosphatasia (HPP), X-linked hypophosphatemia, pycnodysostosis, osteopetrosis, osteoporosis pseudoglioma syndrome (OPPG), osteogenesis imperfecta (OI), and X-linked osteoporosis, involving genes such as COL1A1, COL1A2, ALPL, PHEX, CTSK, LRP5, SERPINF1, IFITM5, CRTAP, and PLS3 [25•]. The gene discussed is PHEX; the disease is osteogenesis imperfecta.