If the kinase that adds the inhibitory phosphate to cdk were defective then the M phase would begin prematurely. Of the many proteins involved in cell cycle control, cyclin-dependent kinase (CDKs) are among the most important. They are a family of multifunctional enzymes that can modify various protein substrates involved in cell cycle progression. M-phase promoting Factor is a complex of a cyclin and cyclin dependent kinase that, when activated, phosphorylates a number of specific proteins needed to intiate mitosis in eukaryotic cells. The cyclin sub-unit regulates the formation of the MPF dimer; the kinase subunit catalyzes the phosphoryation of other proteins to start M phase.