Extrapolating the results from two trials in WHO grade III oligodendroglioma (Bent et al. 2013; Cairncross et al. 2013) further strengthens this recommendation for WHO grade II oligodendroglioma, while the positive second interim analysis of the CATNON trial (Bent et al. 2017,2019) supports this strategy for IDH-mut grade II astrocytomas. The gene discussed is IDH1; the disease is oligodendroglioma.