Meanwhile, neither thymidine synthase expression nor thymidine kinase activity correlated with the growth inhibitory effect of 5-fluorouracil in the analyses examining the association between the growth inhibitory effect of 5-fluorouracil and the expression of thymidine synthase, thymidine kinase, and DPYD in 63 cancer cell lines including 11 breast cancer cell lines [27]. This evidence concerns the gene DPYD and breast cancer.