IDH1 and astrocytoma (excluding glioblastoma): Similarly, homozygous deletion of CDKN2A in IDH-mutant infiltrating astrocytoma confers a grade 4 diagnosis even in the absence of other high grade histological features, as described in the cIMPACT-NOW update 5 [3, 4, 7, 8, 14, 32, 38].