IDH1 and glioma: This interpretation is consistent with the requirement of a wild-type IDH1 allele for D2-HG production [16, 17] and the frequent loss of either wild-type or mutant IDH1 allele in patient-derived xenograft, ex vivo neurosphere culture, and glioma recurrence and progression [11, 16, 18, 19], even though the underlying mechanism of copy number alteration remains unclear.