The first mutation, c.436G>C (p.Gly146Arg) characterized in 3/19 (15.8%) of the Egyptian patients and affecting the residue of the mature domain in CTSK gene, yielded a mature form of a nonfunctional protein and had been previously found in many pycnodysostosis families from diverse ethnic backgrounds: Moroccan Arab, American Hispanic, Tunisian, Algerian, and Egyptian patients [18,19,21,37,38]. Here, CTSK is linked to pycnodysostosis.