- The restriction point (R-point) marks the critical event when a mammalian cell commits to proliferation and becomes independent of growth stimulation. It is fundamental to normal differentiation and tissue homeostasis and appears dysregulated in virtually all cancers. Although the R-point has been linked to various activities involved in G1/S control, the underlying mechanism remains elusive. Using single-cell measurements, here we show that the Rb/E2F pathway functions as a bistable switch to convert graded serum inputs into all-or-none E2F responses. Once turned ON by sufficient serum stimulation, E2F can memorize and maintain this ON state, independent of continuous serum stimulation. We further show that, in both critical dose and timing responses, bistable E2F activation directly correlates with a cell's ability to traverse the R-point.