CTSK and pycnodysostosis: Cathepsin K is encoded by a CTSK gene located on chromosome 1q21.3, with eight exons and seven introns, and a similar organization of cathepsin L and cathepsin S. Mutations on the CTSK gene lead to pycnodysostosis, an autosomal recessive disease characterized by osteosclerosis with increased bone fragility, dysmorphic facial features, and short stature [12].