5-Fluorouracil (5-FU) has been one of the most commonly used drugs in the treatment of various types of cancer (in particular, colorectal and breast) for the past fifty years. 5-FU is a drug for which genetic and epigenetic variation in the drug-metabolizing enzyme (dihydropyrimidine dehydrogenase [DPD]) and the drug target enzyme (thymidyalate synthase [TS]) can influence both toxicity and respon ...