Polo-like kinase (PLK) plays an important role in cell cycle regulation. It is required for bipolar spindle formation, activation of the anaphase-promoting complex/cyclosome, and cytokinesis. Recent work led to the identification of a polo-like kinase kinase (PLKK) that is thought to be responsible for activation of PLK. Recent work (Erikson, et al., 2004) has shown that PLKK is in turn activated by phosphorylation at three sites (Ser482, Ser486 and Ser490). Thus activation of PLK is thought to involve a kinase cascade involving the phosphorylation of Ser482,486,490 in PLKK.