The cell cycle is regulated by activators and inhibitors. Cell cycle progression is driven by proteins called cyclins. cyclin D-CDK4, cyclin D-CDK6, and cyclin E-CDK2 regulate the G1-to-S transition by phosphorylating the Rb protein (pRb). Cyclin A-CDK2 and cyclin A-CDK1 are active in the S phase. Cyclin B-CDK1 is essential for the G2-to-M transition.Reference: Robbins and Cotran Pathologic Basis of Disease; 9th edition; Chapter 1; The Cell as a Unit of Health and Disease