Pycnodysostosis is a genetic lysosomal disease characterized by short stature, increased density of the bones (osteosclerosis/osteopetrosis), and brittle bones. Other features may include underdevelopment of the tips of the fingers with absent or small nails, an abnormal collarbone (clavicle), distinctive facial features including a large head with a small face and chin, underdeveloped facial bones, a high forehead, and dental abnormalities.Pycnodysostosis is an autosomal recessive condition caused by mutations in the gene that codes the enzyme cathepsin K (CTSK) on chromosome 1q21. The diagnosis of pycnodysostosis is based on physical features and X-ray findings. Molecular genetic testing is available. Treatment should address the symptoms found in each patient and may include orthopedic monitoring, treatment of fractures, appropriate dental care, and craniofacial surgery. The disease is MONDO_0009940 (pycnodysostosis). Also known as: Pyknodysostosis.