5-fluorouracil (5-FU) is an anti-metabolite drug that has been widely used since 1957 in various types of cancer such as breast cancer, colorectal cancer, pancreatic cancer, etc. for cytotoxicity; it interferes with the biosynthetic activity by inhibiting thymidylate synthase (TS). The gene discussed is TYMS; the disease is familial pancreatic carcinoma.