IDH1 and central nervous system cancer: Although IDH mutation is retained upon glioma recurrence (Bai et al., 2016; Mazor et al., 2015), mutant IDH1 may convert from driver to passenger (Johannessen et al., 2016), and, in some patients, neither mutant IDH1 nor the oncometabolite 2HG are strictly required for clonal expansion at recurrence (Mazor et al., 2017).