First described in 1962 and said to be the malady of both Toulouse-Lautrec and Aesop (known for his fables) (234–236), pycnodysostosis (MIM265800) is caused by defective enzymatic degradation of organic bone matrix, due to an autosomal recessive mutation in CATK (coding for cathepsin K) (207). Here, CTSK is linked to pycnodysostosis.