This mainly impacts the common diffuse gliomas of adults, which were previously divided into 15 entities but now include only three: astrocytoma, isocitrate dehydrogenase (IDH)-mutant; oligodendroglioma, IDH-mutant and 1p/19q codeleted; and glioblastoma, IDH-wildtype. The gene discussed is IDH1; the disease is astrocytoma (excluding glioblastoma).