By screening the expression of CD and HSV1-tk genes (KDR/CD-TK) in colon cancer cells, KDR promoter expression has been proven to be specific for this type of cells, finding high levels of CD/TK mRNA in SW480 and SW620 (KDR positive human colon adenocarcinoma) which were found to be highly susceptible to 5-FC and ganciclovir prodrugs and with no effect on LS174T cells (KDR negative human colon adenocarcinoma) (138). Here, TKT is linked to colonic neoplasm.