Editing Cell cycle (section)

==== Endogenous ====
[[Image:Signal transduction pathways.svg|500px|thumb|Overview of signal transduction pathways involved in [[apoptosis]], also known as "programmed cell death"]]
Two families of genes, the ''cip/kip'' (''CDK interacting protein/Kinase inhibitory protein'') family and the INK4a/ARF (''In''hibitor of ''K''inase 4/''A''lternative ''R''eading ''F''rame) family, prevent the progression of the cell cycle. Because these genes are instrumental in prevention of [[tumor]] formation, they are known as [[tumor suppressor]]s.

The '''''cip/kip'' family''' includes the genes [[p21]], [[p27 (gene)|p27]] and [[P57 (gene)|p57]]. They halt the cell cycle in G<sub>1</sub> phase by binding to and inactivating cyclin-CDK complexes. p21 is activated by [[p53]] (which, in turn, is triggered by DNA damage e.g. due to radiation). p27 is activated by Transforming Growth Factor β ([[TGF β]]), a growth inhibitor.

The '''INK4a/ARF family''' includes [[p16 (gene)|p16<sup>INK4a</sup>]], which binds to CDK4 and arrests the cell cycle in G<sub>1</sub> phase, and [[p14arf|p14<sup>ARF</sup>]] which prevents p53 degradation.