The key step, the conversion of deoxy-5-fluorouridine (5′-DFUR) into active 5-FU is catalysed by thymidine phosphorylase (TP), which is expressed at elevated levels by the liver and many tumours, allowing capecitabine to be specifically targeted to the site of the cancer, leading to relatively high local concentrations of 5-FU in tumour cells (Bollag and Hartmann, 1980; Walko and Lindley, 2005). The gene discussed is TYMP; the disease is neoplasm.