CDK4 and neoplasm: In the mammalian G1/S transition network, pRB and AP-1 are the tumor suppressor from the family of pocket proteins and the family of transcription factors that mediate mitogenic signals, E2F1 is the transcription factor targeting genes that regulate cell cycle progression, Cyclin D/cdk4,6, cyclin E/cdk2, complexes characterizing the G1- and S- phases.