<em><u>Two groups of proteins, called cyclins and cyclin-dependent kinases (Cdks), are responsible for the progress of the cell through the various checkpoints. ... Cyclins regulate the cell cycle only when they are tightly bound to Cdks.</u></em><em><u>The cell cycle is controlled by a number of protein-controlled feedback processes. Two types of proteins involved in the control of the cell cycle are kinases and cyclins. Cyclins activate kinases by binding to them, specifically they activate cyclin-dependent kinases (CDK).</u></em>
