Linkage Homolog of thymidine Application 5-Ethyl-2′-deoxyuridine (EUdR) is used as a 5-fluorouracil (FU) modulator. EtdUrd may be used to enhance the therapeutic index of 5-FU by reducing the catabolism, prolonging the plasma and intratumoral concentrations of 5-FU, and offering protection to normal organs by increasing the endogenous uridine levels.