DPYD and dihydropyrimidine dehydrogenase deficiency: In order to further determine whether the endogenous DPD substrate thymine may be a more consistent and predictive biomarker for DPD-deficiency, this study aimed to determine the correlation between the baseline endogenous DPD substrate plasma (ratio) concentrations of thymine, uracil, DHU and dihydrothymine (DHT) and the systemic drug exposure of 5-FU and DPD enzyme activity in patients with gastrointestinal cancer treated with intravenous 5-FU-based chemotherapy.