The study reported thickened, shortened bones, and disorganized growth plates with an increased presence of cartilage mimicking that of cathepsin K-deficient mice or patients with pycnodysostosis, a rare genetic disease due to inactivation of cathepsin K. Although the protein level of cathepsin K was higher in MPS-I bones than controls, cathepsin K-related collagenolytic activity was reduced. Here, CTSK is linked to pycnodysostosis.