PLK1 (polo-like kinase 1) is a member of the serine/threonine protein kinase family, cdc5/polo subfamily. PLK1 contains two polo box domains with a predicted molecular weight of 68 kDa. PLK1 has been shown to regulate cdc2/cyclin B through phosphorylation and activation of cdc25c phosphatase. PLK1 is modified by phosphorylation at Threonine 210. PLK1 may also be required for cell division. Depletion of PLK1 results in apoptosis and deregulation of expression of PKL1 is correlated with development of many malignancies.