Capecitabine is converted by carboxylesterase in the liver to 5′-deoxy-5-fluorocytidine (5′-DFCR), by cytidine deaminase in the liver and tumor tissue to 5′-deoxy-5-fluorouridine (5′-DFUR), and by thymidine phosphorylase (TP) to 5-FU in tumor tissue. The gene discussed is CDA; the disease is neoplasm.