(A) The cell cycle involves the following phases: the S-phase, during which DNA is duplicated, and the M-phase (or mitosis), during which chromosome segregation and cell division happen. A gap or G1-phase precedes the S-phase, and the G2-phase happens before the M-phase. Cyclins and cyclin-dependent kinases (CDKs) form a complex that drives progress through the cell cycle: the activity of these complexes (indicated by the width of the red region) starts in G1-phase and increases throughout the cell cycle, until the cyclins are destroyed during the M-phase. Cyclin-CDK complexes drive the G1-to-S transition (cell cycle commitment) by activating the E2F transcription factors and triggering large-scale changes in gene expression. (B) The long-standing model of how E2F-transcription is activated is as follows: initial hypo-phosphorylation (hypo ℗) and gradual inactivation of Rb by cyclin D/CDK leads to an accumulation of cyclin E. This triggers a positive feedback loop, as more cyclin E/CDK results in hyper-phosphorylation (hyper ℗) and complete inactivation of Rb, which activates E2F-dependent transcription. (C) The work by Dowdy and co-workers establishes a new role for cyclin D/CDK in the regulation of Rb activity by demonstrating that mono-phosphorylation (mono ℗) of Rb by cyclin D/CDK in fact activates Rb to repress E2F transcription. Mono-phosphorylated Rb is the active form in dividing cells, and it is throught that un-phosphorylated Rb is involved in exiting from the cell cycle into a resting or ‘G0-phase’ (the thickness of the inhibition arrows reflects strength of Rb activity). Based on these results Dowdy and co-workers suggest that an increase in cyclin D/CDK activity allows cells to enter the G1-phase and be primed for cell cycle progression. Whether mono-phosphorylation prevents exit from the cell cycle or promotes entry into G1 phase is currently unknown (indicated by ‘?’). Hyper-phosphorylation by cyclin E/CDK inactivates Rb and activates E2F transcription, without the involvement of cyclin D/CDK. However, the mechanism responsible for triggering the accumulation of cyclin E/CDK is unknown (possible ways are indicated by ‘?’).