Tumor resistance to 5-fluorouracil (5-FU) is correlated with high levels of TS gene expression in colorectal tumors, primary adenocarcinoma of the stomach and lung cancer.(28) Rarely, unexpected severe toxicity (e.g., stomatitis, diarrhea, neutropenia, and neurotoxicity) associated with 5-fluorouracil has been attributed to a deficiency of dihydropyrimidine dehydrogenase (DPD) activity.(29). This evidence concerns the gene DPYD and neutropenia.