Cathepsin K is a cysteine proteinase expressed predominantly in osteoclasts. Cathepsin K cleaves key bone matrix proteins and is believed to play an important role in degrading the organic phase of bone during bone resorption. Pycnodysostosis, an autosomal recessive osteosclerosing skeletal disorder has recently been shown to result from mutations in the cathepsin K gene. Cathepsin K deficient mice generated by targeted disruption of this proteinase phenocopy many aspects of pycnodysostosis. They display an osteopetrotic phenotype with excessive trabeculation of the bone-marrow space accompanied by an altered ultrastructural appearance of the cathepsin K deficient osteoclasts. These cells also demonstrate an impaired resorptive activity in vitro. In contrast to other forms of osteopetrosis, which are due to disrupted osteoclastogenesis, cathepsin K deficiency is associated with an inhibition of osteoclast activity. Taken together the phenotype of cathepsin K knockout mice underlines the importance of this proteinase in bone remodelling.