Retinoblastoma-associated protein (RB1) is a key regulator of entry into cell division that acts as a tumor suppressor. RB1 acts as a transcription repressor of E2F1 target genes. The underphosphorylated, active form of RB1 interacts with E2F1 and represses its transcription activity, leading to cell cycle arrest. Defects in RB1 are the cause of childhood cancer retinoblastoma (RB). RB is a congenital malignant tumor that arises from the nuclear layers of the retina. RB1 is e, Xenopus/Amphibian,pressed in the retina.