Source:http://linkedlifedata.com/resource/umls/id/C1517344
NCI: The cell cycle transition from G1 to S phase is a key regulatory point in the cell cycle. This transition is regulated by the checkpoint kinase cdk2 that activates the G1 to S transition when it is associated with cyclin E. Cdk2/Cyclin E causes the G1 to S transition through phosphorylation of the tumor suppressor Rb, releasing the transcription factor E2F-1. Other pathways acting through Rac, Ras, and Rho also regulate the G1 to S transition. Ras regulates cyclin D1 expression to affect the G1 to S transition. Transforming forms of Ras or Raf induce cyclin D1 expression and cause early entry into S phase. Signaling from Ras to Raf to MEK to ERKs induces Cyclin D1 expression, allowing Cyclin D1 to complex with Cdk4 and Cdk6 and phosphorylate Rb. Rac-1 and PAK appear to induce Cyclin D1 expression and induce the G1 to S transition primarily through activation of NF-kB to activate the Cyclin D1 promoter. Rho activates cdk2 and also inhibits p21 and p27 to induce cyclin D1 and stimulate the G1 to S transiti