Thymidine inhibits the incorporation of 5-fluoro-2'-deoxyuridine to DNA of mouse mammary tumour.

5-Fluoro-2'-deoxyuridine is incorporated into DNA of mouse breast tumour in vivo. The incorporation is inhibited by thymidine. Part of the fluorodeoxyuridine is cleaved to fluorouracil and is incorporated into RNA. This incorporation is enhanced by thymidine. The result suggests that the major mechanism of action of the fluorouracil is due to its incorporation into RNA.