PLK1 and cancer: Consistent with these observations, differential gene analysis showed that the expression of E2F target genes such as those encoding Aurora kinase 1, Polo-like kinase 1, Survivin (BIRC5) and the cell cycle regulators CDK1, cyclin-A1 and cyclin-E2 were 3- to 4-fold up-regulated in malignant tumors (Figures 1D-1E, Table S1).