MAPK1 and Osteochondroma: While it is clear that Erk2 is not deleted in the majority of the chondrocytes in the DKOCD4 mice, osteochondromas can form when only a proportion of chondrocytes lack critical regulators such as ptpn11 (SHP2) or Ext1. Mice with deletion of these genes in 10–50% of chondrocytes developed cartilaginous tumors composed of a mix of wild-type and mutated chondrocytes (18, 19).