Mad3 and its interaction with Cdc20. A. Schematic drawing of Saccharomyces cerevisiae Mad3 (homolog of human BubR1). Mad3 contains several APC/C degradation motifs: a destruction box (D box) and two KEN boxes. B. Three possible mechanisms of Mad3 binding to Cdc20. The monovalency model proposes the cooperative binding of multiple motifs into a single docking site in Cdc20. The multivalency model proposes that the multiple degradation motifs of Mad3 bind to different docking sites in Cdc20. In the shared-binding model, the degradation motifs of Mad3 bind to different sites in both Cdc20 and APC/C.