Such studies are of particular importance for the use of 5-FU in the era of personalized medicine because a significant percentage of cancer patients are deficient in 5-FU degradation due to polymorphisms in DPYD [30], the gene encoding dihydropyrimidine dehydrogenase (DPD), which catalyzes the initial step in 5-FU degradation. The gene discussed is DPYD; the disease is cancer.