For example, capecitabine, under the brand name Xeloda, is a widely used chemotherapeutic drug in the treatment of breast cancer, gastric cancer, and colorectal cancer.124 Inside the human body, capecitabine needs to be metabolized into 5-FU, a thymidylate synthase inhibitor which interrupts the process of pyrimidine thymidine synthesis.125 This process is mediated through three enzymes, carboxylesterases (CES), cytidine deaminase (CDA), and thymidine phosphatase (TYMP), as detailed in Fig. 5a. The gene discussed is CDA; the disease is colorectal cancer.