IDH1 and central nervous system cancer: Mutations in the IDH3 gene have not been reported in connection with tumors; therefore, in this article the term IDH mutation refers only to IDH1 and IDH2. The most common amino acid substitution in glioma is the replacement of arginine with histidine in the IDH1 gene (IDH1R132H; Parsons et al. 2008; Yan et al. 2009; Borger and Zhu 2012; Hirata et al. 2015, Table 1).