The major molecular mode of action of the cytotoxic drug 5-fluorouracil (5-FU) is generally considered to result from thymidylate synthase inhibition. Recent findings relating to the function of the human uracil-5 methyltransferase (U5MT), TRMT2A, and its interaction with 5-FU metabolites incorporated within tRNAs, lead to an additional hypothesis that is proposed here. The major molecular mode of action of the cytotoxic drug 5-fluorouracil (5-FU) is generally considered to result from thymidylate synthase inhibition. Recent findings relating to the function of the human uracil-5 methyltransferase (U5MT), TRMT2A, and its interaction with 5-FU metabolites incorporated within tRNAs, lead to an additional hypothesis that is proposed here.