CDKN2B and astrocytoma (excluding glioblastoma): Therefore, a diagnosis of astrocytoma, IDH mutant, or CNS WHO grade 4 requires either morphologic features of a glioblastoma, namely necrosis or microvascular proliferation, or homozygous deletion of CDKN2A and/or CDKN2B (see Figure 1) [1,14].