Capecitabine is a pro-drug that is absorbed from the intestine and metabolised into 5′-deoxy-5-fluorocytidine (5′-DFCR) in the liver by carboxylesterase, then further metabolised to 5′-deoxy-5-fluorouridine (5′-DFUR) in the liver or tumours by cytidine deaminase, and finally converted to fluorouracil (5′-FU)—its active form—by thymidine phosphorylase (TP), which is highly expressed in tumours27 (Supplementary Fig. 9a). The gene discussed is CDA; the disease is neoplasm.