Rb (RB1)

What it is. Gatekeeper of the G1→S cell-cycle transition; binds E2F transcription factors to keep cells from copying DNA at the wrong time.

Why it matters here. When Rb is inactivated, E2F is unleashed and cells proliferate despite checkpoints.

Notes. Type: tumor suppressor; Pathways: Cyclin/CDK, p16^INK4a–Rb–E2F axis.