Since about 80% of 5-FU is catabolized by dihydropyrimidine dehydrogenase (DPYD), the initial and rate-limiting enzyme of pyrimidine catabolism, only a small part of the 5-FU can inhibit thymidylate synthase (TS) so as to cause cancer cell death via thymineless death [9,10]. Here, DPYD is linked to cancer.