Activator protein that regulates the ubiquitin ligase activity and substrate specificity of the anaphase promoting complex/cyclosome (APC/C). During telophase and in the subsequent G1 phase of the cell cycle, recognizes and binds proteins containing a destruction box (D-box) and an additional degradation signal termed the KEN box including ASE1, CDC20, the B-type cyclins CLB2 and CLB3, the polo-like kinase CDC5 and HSL1, and recruits them in a C-box-dependent manner to the APC/C for ubiquitination and subsequent proteolysis. Required for exit from mitosis, cytokinesis and formation of prereplicative complexes in G1. Probably is the target of a BUB2-dependent spindle checkpoint pathway. Associates with the APC/C complex. Interacts with CLB2, CLB3, CDC5, HSL1, MSN5 and PSE1. Nuclear import and export are mediated by the importin PSE1 and the exportin MSN5. The C-box is required for the association with the APC/C complex. Phosphorylated at multiple sites by CDC28, probably in its CLB5 bound form, in S, G2 and M phase of the cell cycle, thereby blocking the association of CDH1 to the APC/C and promoting nuclear export of CDH1 by MSN5. Dephosphorylated and activated by CDC14 in late anaphase, which may be necessary for PSE1-dependent nuclear localization. Belongs to the WD repeat CDC20/Fizzy family.