To address if the p21 status is a direct factor for the efficacy of Plk1 inhibitors, we have chosen the isogenic colon cancer cell lines HCT116 p21+/+ and HCT116 p21−/−, as they contain comparable cellular context except the p21 status and are very well characterized [22]. This evidence concerns the gene CDKN1A and malignant colon neoplasm.