sentences:  Once Cdk4 is activated and is bound with Cyclin D, the Cyclin D/Cdk4 complex phosphorylates Retinoblastoma protein (pRb).Once the Retinoblastoma protein has been phosphorylated, E2F is released.The released E2F is then free to act as a transcription factor and it subsequently binds to DNA promoter regions and activates the expression of proteins required in the next stages of the cell cycle and in DNA replication.