Pycnodysostosis is an inherited bone disorder that causes abnormally dense, brittle bones. This is due to a defective enzyme called cathepsin K. Because the bones are so brittle, bones of people with pycnodysostosis break easily. These breaks will typically occur in a person's legs, feet, jaw, and collar bone. Individuals with pycnodysostosis have short stature, as well as short fingers, a wide open soft spot on the skull, and characteristic facial features. Currently, there is no treatment that corrects the bone problems in individuals with pycnodysostosis; however, growth hormone replacement can be used to improve the height of these individuals. Also, plastic surgery can be used to correct the facial features. The life expectancy of people with this disorder is normal or nearly-normal. Pycnodysostosis is caused by pathogenic variants in the CTSK gene.