RB1 and central nervous system cancer: Retinoblastoma protein (Rb) mutation was identified in 20% of malignant gliomas (Behin et al., 2003), although gliomas may also contain mutations in other molecules involved in Rb signaling, including cyclin-dependent kinase (CDK) and the cell cycle regulator cyclin-dependent kinase inhibitor 2A, multiple tumor suppressor 1 (p16INK4A).