Growth factors are required for the Gap1 (G1) phase to progress to the S phase. Growth factors stimulate the synthesis of a cell cycle protein called cyclin D. In turn, cyclin D forms a complex with cyclin-dependent protein kinase 4 (CDK4) called the Cyclin D-CDK4 complex.
Cells in G1 phase do not automatically progress to the S phase. A checkpoint protein called retinoblastoma (Rb or pRb) functions as a negative regulator of the cell cycle.
To ensure cell cycle progression, Rb ought to be inactivated. Rb inactivation is carried out by the cyclin D-CDK4 complex.
