The protein encoded by the RB1 gene is a negative regulator of the cell cycle and was the first tumor suppressor gene identified. The active, hypophosphorylated form of the protein binds transcription factor E2F1. RB1 alterations including mutations and deletions are seen in up to 11% of glioblastomas. It has been suggested that patients with RB1-deficient tumors do not respond to cyclin-dependent kinase (CDK) inhibitors. The predictive and prognostic significance of RB1 mutations in glioblastoma needs to be further studied.