EXT1 and chondrogenic neoplasm: However, the notion that cartilage tumors can arise from a mixture of mutant and non-mutant cells is supported by the observation that enchondromas in patients with IDH1 mutations contain both wild-type and IDH1-mutant chondrocytes [7], and that exostoses in Ext1-deficient mice contain both Ext1-null and wild-type chondrocytes [32].