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.
For a cell to progress from the Growth 1 phase into the Synthesis phase of its cycle, which includes the Start or G1/S checkpoint, it is essential that the Cyclin D/Cdk4 complex be involved in this transition.