The Mammalian Cell Cycle Showing the G1, S, G2, and M Phases along with the Predominant Cyclin–CDK Activities Associated with Each Phase (Top Panel)

<p>The lower panel shows the position of the R point (R) which subdivides the G1 phase into G1-pm (post-mitosis) and G1-ps (pre-S-phase). Quiescent or non-dividing cells have to be exposed to continuous growth-factor stimulation up until the R point in order to commit to entry into S-phase. After R and a finite induction period, cyclin E/CDK2 activity increases (shown by the dashed curve labelled E) as reported by Ekholm et al. [<a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.0030152#pcbi-0030152-b006" target="_blank">6</a>].