EXT1 and Osteochondroma: In good correlation, we and others showed that while single heterozygous Ext1+/- or Ext2+/- mice were largely normal, compound heterozygous Ext1+/-;Ext2+/- and conditional Ext1f/f;Col2CreER-null mice (both producing less HS than single heterozygous mice) displayed multiple osteochondromas and closely mimicked human HME [20–24].