sentences:  As its name would suggest, the cyclin-dependent kinase is only active and able to phosphorylate its substrates when it is bound by the corresponding cyclin.The Cyclin D/Cdk4 complex is integral for the progression of the cell from the Growth 1 phase to the Synthesis phase of the cell cycle, for the Start or G1/S checkpoint.Under non-dividing conditions (when the cell is in the G0 phase of the cell cycle), Retinoblastoma protein (Rb) is bound with the E2F transcription factor.