Apcin is a potent, competitive inhibitor of the APC/C-Cdc20 E3 ligase, binding Cdc20 to block substrate recognition and ubiquitination. This inhibition arrests cells in mitosis and is synergistically enhanced by Ts-Arg-OMe, making it a valuable tool for studying cell cycle regulation, mitotic exit, and aneuploidy in both cellular and biochemical assays.