IDH2 and glioma: It appears that the removal of the arginine, rather than the substitution of a specific residue, is the critical feature of this mutation, although the most frequent replacement observed is a histidine residue (R132H).51, 52, 93, 98 Mutations in IDH2 in gliomas occur at a much lower frequency (0–6%) than IDH188 in the paralogous amino acid residue—R140—but also at R172.