Editing G1 phase (section)

===Biochemical regulators===
{{Main|Cyclin E1}}

During G<sub>1</sub> phase, the G<sub>1</sub>/S cyclin activity rises significantly near the end of the G<sub>1</sub> phase.

Complexes of cyclin that are active during other phases of the cell cycle are kept inactivated to prevent any cell-cycle events from occurring out of order. Three methods of preventing Cdk activity are found in G<sub>1</sub> phase: [[Retinoblastoma protein|pRB]] binding to [[E2F]] family transcription factors downregulate [[Gene expression|expression]] of S phase cyclin genes; [[anaphase-promoting complex]] (APC) is activated, which targets and degrades S and M cyclins (but not G<sub>1</sub>/S cyclins); and a high concentration of Cdk inhibitors is found during G<sub>1</sub> phase.<ref name=Morgan />