The presence of a CDKN2A/B homozygous deletion isindicative of a high-grade astrocytoma, even in the absence of high-gradehistological findings, such as microvascular proliferation and necrosis.Low-grade IDH-mutant astrocytomas can undergo late transformation to high grade(Figure 1). The gene discussed is CDKN2A; the disease is astrocytoma (excluding glioblastoma).