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.