5-fluorouracil (5-FU) is a common chemotherapeutical drug. It exerts its antitumor effect through competitive thymidylate synthase (TS) inhibition. Thymidylate synthase (TS) catalyses deoxyuridine-5ЎЇ-monophosphate (dUMP) to 2ЎЇ-deoxythymydine-5ЎЇ-monophosphate (dTMP).

The cancerous cells of some individuals with breast cancer lack expression of two cell surface proteins, the estrogen and progesterone receptors, and do not express increased amounts of HER2. Individuals with such breast cancer (known as triple-negative breast cancer) do not respond to treatment with commonly used chemotherapeutic drugs and their prognosis is relatively poor.