The protein encoded by RB1 (RB transcriptional corepressor 1) is a negative regulator of the cell cycle and was the first tumor suppressor gene found. The encoded protein also stabilizes constitutive heterochromatin to maintain the overall chromatin structure. The active, hypophosphorylated form of the protein binds transcription factor E2F1. Defects in RB1 are a cause of childhood cancer retinoblastoma (RB, bladder cancer, and osteogenic sarcoma.