CTSK and pycnodysostosis: For example, osteogenesis imperfecta has shown the importance of bone collagen matrix quality; Van Buchem disease, Hajdu-Cheney syndrome and autosomal recessive osteopetrosis have revealed important signalling pathways (namely Wnt, Notch and RANK-RANKL-OPG) that regulate bone remodelling; and pycnodysostosis has given insight into the pivotal action of cathepsin K in osteoclast function.