CTSK and pycnodysostosis: The identification of CTSK defects as the cause of pycnodysostosis in 1996, and subsequent studies of its function in bone resorption, has led to the development of cathepsin K inhibition as a promising therapeutic approach for osteoporosis 20 years later, highlighting the importance of recognising molecular mechanisms in order to advance medical care and the fast pace of translation in this burgeoning field (34).