EXT1 and Osteochondroma: The role of Ptpn11 in skeletal development and cartilaginous tumor formation has been investigated in mice [21–24]; interestingly, different mechanisms underlie the development of chondroma-like lesions in Shp2-deficient mice, enchondroma-like lesions in Pthr1 R150C mutant mice, and osteochondroma-like lesions in Ext1-deficient mice [24].