Most of them cause known monogenic disorders, such as osteogenesis imperfecta (COL1A1/COL1A2), pycnodysostosis (CTSK), X-linked osteoporosis (PLS3), osteopetrosis, X-linked hypophosphatemia (PHEX), and osteoporosis pseudoglioma syndrome (LRP5) [43]. The gene discussed is COL1A2; the disease is pycnodysostosis.