Fig. 1. Two early models to explain how cyclin activates MPF (Minshull et al., 1989b), published a few months before MPF was identified as a stoichiometric complex comprising one molecule of Cdc2 and one molecule of cyclin B. The upper cartoon shows cyclin as binding and thereby removing a hypothetical anti-MPF subunit, thus activating Cdc2. According to this diagram, anti-MPF acts stoichiometrically, but both anti-MPF and cyclin might act catalytically, for example to alter the phosphorylation state of Cdc2. In the lower model, cyclin modifies the hypothetical inhibitor, which allows Cdc2 dephosphorylation.