PLK (Polo-like kinases) are important regulators of the cell cycle. PLKs are involved in the formation of and the changes in the mitotic spindle and in the activation of CDK/cyclin complexes during the M-phase of the cell cycle. PLKs are a family of conserved serine/threonine kinases involved in the regulation of cell cycle progression through G2 and mitosis. Mammalian polo-like kinases include Plk1 (Xenopus Plx1), Plk2/Snk (Xenopus Plx2), Plk3/Prk/FnK (Xenopus Plx3), Plk4/Sak and Plk5. PLKs are involved in aspects of mitosis that include mitotic entry and exit and cytokinesis.PLKs mediate G2/M transitions, activation of cdc25 and mitotic processes including centrosome maturation, bipolar spindle formation, activation of the anaphase-promoting complex (APC), chromosome segregation, and actin ring formation (cytokinesis). Plk1 is involved in the regulation of key steps during cell division, DNA damage repair pathways, apoptosis, and the progression of the cell cycle. Plk3 is a multifunctional stress response protein that responses to signals induced by DNA damage and/or mitotic spindle disruption.