PIK3CA and oligodendroglioma: As there have been many studies on genomic sequencing of oligodendrogliomas already, we only further studied the ALT-positive cases by targeted DNA sequencing (Supplementary Table 3), including genes described to be associated with poorer prognosis in oligodendrogliomas, mostly in Grade 3 cases, namely CIC, FUBP1, TCF12, PIK3CA, and NOTCH1 (Supplementary Tables 4 and 5).1 ATRX mutation as expected was found only in 1 case and no p53 mutation was found.