Capecitabine is metabolised to 5-FU through a number of enzymatic steps. It is the first rationally designed drug that is based upon the high concentration of thymidine phosphorylase (TP) in many human tumors compared to normal tissue. TP is the last step in the conversion of capecitabine to 5-FU and seems to be the limiting factor for the activation. Capecitabine may to some extent mimic continues 5-FU infusion as opposed to bolus 5-FU. A number of small investigations have indicated that patients with 2R/2R TS polymorphism have a higher response rate than heterozygous patients.