Overview of natural 2′-deoxynucleosides and selected nucleoside analogs. Thymidine (T, 1), 2′-deoxycytidine (dC, 2), 2′-deoxyadenosine (dA, 3), 2′-deoxyguanosine (dG, 4), 3′-azido-3′-deoxythymidine (AZT, 5), 2′,3′-dideoxy-5-fluoro-3′-thiacytidine (FTC, 6), ganciclovir (7), 3-deoxythymidine (ddT, 8). Grey-shaded areas are marking structural and functional modifications in the prodrugs.