The APC/C is activated by either Cdc20 or Cdh1. While both activators associate with the APC/C, they do so at different points in the cell cycle and their binding is regulated differently (see Zachariae and Nasmyth, 1999). Cdc20, whose protein levels increase as cells enter into mitosis and decrease upon mitotic exit, only associates with the APC/C during M phase. Cdh1 associates with the APC/C in G1. This interaction is inhibited at other times by Cdk1 phosphorylation.