Around 20%-30% of patients treated with fluoropyrimidines develop severe treatment-related adverse events (AEs). These are mainly caused by deficiency of dihydropyrimidine dehydrogenase, its main metabolizing enzyme. The Patients treated with standard-of-care fluoropyrimidine who were pretreatment From 3,748 patients, we found 13 patients carrying heterozygous In this study, the clinical consequences of carrying the