The retinoblastoma gene product is bound in the nucleus in early G1 phase.

The product of the retinoblastoma susceptibility gene (pRB) exerts its growth-regulatory effects during the G1 phase of the cell cycle, where all pRB present has been assumed to be in the underphosphorylated form. We demonstrate here that pRB is underphosphorylated and firmly bound in the nucleus only in early G1 phase. All G0 cells contain bound, underphosphorylated pRB. The duration of the cell cycle and of the G1 phase seems to be determined by the time during which pRB is underphosphorylated and bound in the nucleus. The observed time lag between the phosphorylation and release of pRB in the G1 phase and entry into S phase was 6.5 h and independent of the G1 transit time. The data suggest that pRB is not directly involved in initiation of DNA replication.