CDK6 and glioblastoma: Frequent amplification events found in GBM concern chromosome 7 [EGFR/ MET proto-oncogene (MET)/ cyclin dependent kinase 6 (CDK6)], chromosome 12 [cyclin dependent kinase 4 (CDK4)/, mouse double minute 2 homolog (MDM2)], and chromosome 4 (PDGFRA).