The distinction of two grades (2 and 3) of IDH-mutant, 1p/19q-codeleted gliomas remains controversial and, accordingly, watch-and-wait strategies after complete resection can also be considered for younger patients (<40 years of age) with WHO grade 3 tumours, specifically for those without homozygous CDKN2A/B deletion, although only after gross total resection and in the absence of neurological deficits. The gene discussed is IDH1; the disease is central nervous system cancer.