Cyclin proteins are regulatory proteins that play an important role in cell cycle regulation. There are two types of cyclin proteins, G1 cyclin and mitotic cyclin. Cyclins act as activating proteins that bind to Cdks (Cyclin dependent protein kinases) and form cyclin-Cdk complex. When G1 cyclin binds Cdk, Cdk sends signal to the cell to leave G1 phase and to enter S phase. When mitotic cyclin binds to Cdk, the clell leaves the G2 phase and enters mitosis.