As demonstrated in the two examples, tissue can be highly heterogeneous and regions of normal brain or myeloid infiltration can often affect the signal of chromosomal deletion, especially in morphologically lower-grade glioma in which the CDKN2A/B status is crucial for WHO grading and, even more with recent breakthroughs, currently restricted to specific grade21 for therapy decision. The gene discussed is CDKN2A; the disease is glioma.