Apcin, a ligand of Cdc20, is a potent and competitive anaphase-promoting complex/cyclosome (APC/C(Cdc20)) E3 ligase activity inhibitor. Apcin competitively inhibits APC/C-dependent ubiquitylation by binding to Cdc20 and preventing substrate recognition. Apcin occupes the D-box-binding pocket on the side face of the WD40-domain and can prolong mitosis.