On the other hand, mice in which Ext1 was specifically disrupted for chondrocytes and the limb bud, Ext2 heterozygous mice, and compound Ext1+/−/Ext2+/− mice display severe skeletal defects with cartilage differentiation and chondrocyte maturation, and these defects resembled an autosomally dominant inherited genetic disorder, human hereditary multiple exostoses [128–132]. This evidence concerns the gene EXT2 and hereditary multiple exostoses.