IDH1 and central nervous system cancer: Similarly, hypomethylation occurs during the progression of IDH-mutant low-grade glioma (IDH-mutant LGG) to secondary (IDH-mutant) GBM (9), and during IDH-mutant LGG (grade II/III) recurrence, where G-CIMP–high tumors progress to G-CIMP–low tumors with hypomethylation in the CIMP genes (9, 10).