 The use of TS inhibitors has become a main focus of using TS as a drug target.The most widely used inhibitor is 5-fluorouracil (5-FU), which acts as an antimetabolite that irreversibly inhibits TS by competitive binding.However, due to a low level of 5-FU found in many patients, it has been discovered that in combination with leucovorin (LV), 5-FU has greater success in down regulating tumor progression mechanisms and increasing immune system activity.