This elevated risk of toxicity is due to a lower level of enzyme in normal tissue cells which means that 5-FU cytotoxic characteristics more easily harm benign cells, but 5-FU more effectively acts as an antimetabolite in tumor cells due to lower levels of intratumoral TYMS mRNA. An alternate TYMS mutation, not found in our patient, manifests as a six base-pair (bp) variation in the 3’-untranslated region of TYMS. Here, TYMS is linked to neoplasm.