The RB1 protein is a negative regulator of the cell cycle and was the first tumor suppressor gene found. The active, hypophosphorylated form of the protein binds transcription factor E2F1. Defects in this gene are a cause of childhood cancer retinoblastoma (RB), bladder cancer, and osteogenic sarcoma - See more at: http://www.cancerindex.org/geneweb/RB1.htm#sthash.uzBqWCrJ.dpuf