CDKN2A and glioblastoma: Most primary glioblastomas and recurrent glioblastomas possess at least one defect along the p53- p16Ink4A-p15Ink4b-p14ARF-MDM2 axis and these abnormalities have been implicated in the proliferation, invasion, migration, apoptotic escape, stem cell properties, drug resistance, and response to therapies of GBM [10,13].