In mice, homozygous SHP2 LOF mutations in COL2α1-expressing chondroid cells and the cathepsin K-expressing (CTSK+) groove of Ranvier cells produce enchondromas (cartilaginous masses formed inside bone) and exostoses (outgrowth of cartilage capped masses on bone surfaces) similar to those found in metachondromatosis patients.35,37,39,58 In patients, the cell-type(s) in which a second hit mutation causes cartilage lesions is unknown. This evidence concerns the gene CTSK and metachondromatosis.