CDK4 and glioblastoma: Altogether these results suggest that overexpression of one or more of the four genes investigated (EGFR, CDK4, MDM4, and PDGFRA) is a hallmark of GBM, which cannot be fully explained on the basis of genetic amplification of the corresponding genes, even when gains of chromosome 7 (in the absence of EGFR gene amplification) were also considered.