Capecitabine is converted to 5-FU preferentially in tumor tissue via a three-step enzymatic cascade, firstly converted to 5′-deoxy-fluorocytidine by hepatic carboxylesterase in the liver; secondly converted to 5′-deoxy-5-fluorouridine by cytidine deaminase in the liver and tumor tissues; finally converted to 5-FU at the tumor site by the tumor-associated angiogenic factor thymidine phosphorylase, thereby minimizing the exposure of normal tissue to 5-FU [18]–[20]. This evidence concerns the gene CDA and neoplasm.