Thymectacin, a phosphoramidate derivative of (E)-5-(2-bromovinyl)-2'-deoxyuridine, is a novel small molecule anticancer agent. Thymectacin is selectively active against tumor cells expressing high levels of thymidylate synthase (TS), a critical enzyme in DNA biosynthesis. Thymectacin was as efficacious as irinotecan, a drug recently approved for the treatment of 5-fluorouracil-resistant colon cancer.