Cdc2 lies at the heart of the eukaryotic cell cycle. We now know that it is active only when associated with a cyclin partner — indeed it is the defining member of a family of cyclin-dependent kinases (CDKs). Fifteen years ago things were not so clear. The importance of Cdc2 as a conserved,cell cycle control gene was evident; however, the connection between Cdc2, the periodically degraded cyclins and the elusive maturation-promoting factor(MPF) first described in 1971 remained to be established. In a Commentary onp. 2461, Marcel Dorée and Tim Hunt look back at the series of experiments that made this connection. Purification of MPF by several groups revealed that it contains Cdc2, and Cdc2 was subsequently shown to co-precipitate with cyclins. There was then no precedent for a kinase that required an accessory subunit; the cyclin was instead thought to dissociate an inhibitory subunit from the kinase. Studies in which the M-phase-specific kinase was purified to homogeneity, however,revealed that the active enzyme is a Cdc2-cyclin-B heterodimer that has MPF activity, re-defining Cdc2 as CDK1.