-phase makes an important checkpoint controlled by a bistable regulatory circuit based on an interplay of the retinoblastoma protein pRB, denoted by A (the so-called tumour suppressor, HumanCyc:HS06650) and the retinoblastoma-binding transcription factor , denoted by B (a central regulator of a large set of human genes, HumanCyc:HS02261). This evidence concerns the gene RB1 and neoplasm.