PLK1 phosphorylates CDC25C on serine residue S198. In addition to catalytically activating CDC25C, PLK1-mediated phosphorylation also results in the nuclear accumulation of CDC25C (Toyoshima-Morimoto et al. 2002). It has been shown that Xenopus polo homolog, Plx1, directly phosphorylates and activates Cdc25C, which in turn dephosphorylates and activates Cdc2. This step is critical for the onset of mitosis. Since Plx1-dependent Cdc25C phosphorylation occurs in the absence of Cdc2 activity, it is likely that Plx1 is a triggering kinase, which leads to the activation of Cdc2 and therefore the normal onset of mitosis (Kumagai and Dunphy 1996).