In the context of cancer, fluoropyrimidine-based chemotherapy drugs, including Capecitabine and 5-fluorouracil (5FU), have been used widely for decades to treat several solid tumour types in either the adjuvant or palliative setting.1 2 Whilst these treatments are usually well received by most patients, with 10–20% experiencing mild toxicity, around 1–5% of patients experience severe or life-threatening toxicity due to significantly impaired function of an enzyme, dihydropyrimidine dehydrogenase (DPYD), that normally acts to catabolise 5-FU [1–3]. This evidence concerns the gene DPYD and cancer.