PLK1 and cancer: Furthermore, the overexpression of PLK-1 is strongly associated with many types of human cancers because upregulated PLK-1 causes the inactivation and/or degradation of tumor suppressor gene p53 in a G2-and S-phase-expressed1 (GTSE1) and Topo-1 binding protein- (TOPORS-) dependent manner, respectively [9, 10].