IDH1 and central nervous system cancer: Instead, the vast majority of known IDH mutations in gliomas are heterozygous missense mutations at codon R132 of IDH1 (>90%) or, much less commonly, at the synonymous codon R172 of IDH2 (<3%), which result in substitution of a highly conserved arginine residue involved in substrate coordination at the active site [2,3,4].