Figure 1. Cdk stimulates its own activity by several feedback loops, ultimately leading to mitotic entry. In the presence of DNA damage, Cdk also drives production of p21, which reduces Cdk activity and eventually leads to activation of APC/C-Cdh1. Active APC/C-Cdh1 degrades mitotic inducers and forces cell-cycle exit and senescence. Thus, after DNA damage, the signalling that if unchecked would drive mitotic entry ensures timely cell cycle exit and senescence.