CDKN2A and glioblastoma: The most common alteration of this pathway is homozygous deletion of p15 and p16, which is present in 50% of tumors, as well as mutations in CDKN2A. Amplification/overexpression of CDK4 is detected in 15–20% of GBM, while homozygous deletion/mutation of RB1 is present in 7.6% of GBM.