Polo-like kinases are important regulators of cell cycle progression. PLK1 is a highly conserved Ser/Thr kinase that has essential roles in the formation of mitotic bipolar spindles (van Vugt et al., 2004). Deregulated expression of PLK?s is detected in many types of cancer and associated with oncogenesis (Takei et al., 2005). It has been proposed that PLK1 function is altered at different stages of mitosis through consecutive phosphorylation events at Ser137 and Thr210 (van de Weerdt et al., 2005). Anti-Phospho-Thr210 PLK1 Western blot of rat synaptic membrane showing specific immunolabeling of the ~66 k PLK protein phosphorylated at Thr210 (control). The phosphospecificity of this labeling is shown in the second lane (lambda-phosphatase: (-Ptase). The blot is identical to the control except that it was incubated in (-Ptase (1200 units for 30 min) before being exposed to the phospho-Thr210 PLK antibody. The immunolabeling is completely eliminated by treatment with (-Ptase.