Cell cycle progression is controlled at a point late in G1 designated Start. Passage through Start requires the activity of the cyclin-dependent protein kinase Cdc28. Transition from G1 to S phase requires the association of Cdc28 with members of the G1 cyclin family. Exit from mitosis and initiation of the next cell cycle requires a complex of proteins designated the anaphase-promoting complex (APC). This complex consists of two proteins,Cdc16 and Cdc27 (also referred to as Snb1), which are involved in limiting DNA replication to once per cell cycle. Cdc23, another component of the APC, is required for both entering and exiting anaphase, and is important for the proper separation of sister chromatids. The APC is thought to be stabilized by Cdc26 (also known as Scd26). In addition to these APC proteins, Cdc5 is also required for completion of mitosis. In contrast, Cdc20 acts as a DNA-damage induced checkpoint, preventing mitosis when DNA damage has occurred..