DPYD and neoplasm: Among 5-FU-related enzymes, DPD, a rate-limiting enzyme involved in the degradation of pyrimidine base and pyrimidine-based antimetabolites, is well known to play a critical role in tumour responses and the general toxicity of 5-FU.7 In the human DPD gene (DPYP), 39 mutations and polymorphisms have been identified,8, 9 and patients with DPD-deficient mutations have been shown to suffer severe toxicity when treated with 5-FU.10, 11 On the other hand, overexpression of DPD is associated with the rapid metabolism of 5-FU resulting in its deactivation and weak clinical response.