Serine/threonine-protein kinase PLK1, also known as polo-like kinase 1 (PLK-1) or serine/threonine-protein kinase 13 (STPK13), is an enzyme that in humans is encoded by the PLK1 (polo-like kinase 1) gene. During interphase, PLK1 localizes to centrosomes. In early mitosis, it associates with mitotic spindle poles. A recombinant GFP-PLK1 protein localizes to centromere/kinetochore region, suggesting a possible role for chromosome separation. Plk1 is an early trigger for G2/M transition. Plk1 supports the functional maturation of the centrosome in late G2/early prophase and establishment of the bipolar spindle. Plk1 phosphorylates and activates cdc25C, a phosphatase that dephosphorylates and activates the cyclinB/cdc2 complex. Plk phosphorylates and activates components of the anaphase-promoting complex (APC).