RB1 and glioblastoma: Gene networks implicated in GBM pathogenesis commonly include growth factor signaling (PI3K) and tumor suppressor (p53, Rb) pathways as well as those involved in telomere regulation.2–4 Although GBM is the most common malignant primary central nervous system (CNS) tumor in adults,5 incidence of primary lesions below the tentorium is low.