Proteolysis of mitotic cyclins depends on a multisubunit ubiquitin-protein Ligase, the anaphase promoting complex (APC), Proteolysis commences during anaphase, persisting throughout G(1) until it is terminated by-cyclin-dependent kinases (CDKs) as cells enter S phase. Proteolysis of mitotic cyclins in yeast was shown to require association of the APC with the substrate-specific activator Hct1 (also called Cdh1). Phosphorylation of Hct1 by CDKs blocked the Hct1-APC interaction. The mutual inhibition between APC and CDKs explains how cells suppress mitotic CDK activity during G(1) and then establish a period with elevated kinase activity from S phase until anaphase.