 The 3' carbon of the ribose ring of 2'-deoxycytidine has been replaced by a sulfur atom because it had greater anti-HIV activity and is less toxic than the positive enantiomer.Next in line was 2',3'-dideoxy-5-fluoro-3'-thiacytidine (Emtricitabine, FTC) which is
a structural homologue of lamivudine.The structural difference is the 5-fluoro-modification of the base moiety of lamivudine.