Many attempts have been made to synthesize structurally novel nucleoside derivatives in order to identify effective compounds for the treatment of tumors and virus-caused disease. At our laboratories, as part of our efforts to synthesize 4′-thionucleosides, we have identified and characterized biologically active nucleosides. During the course of our synthetic study, we developed the Pummerer-type thioglycosylation reaction. As a result, we synthesized a potent antineoplastic nucleoside, 1-(2-deoxy-2-fluoro-β-D-4-thio- arabino -furanosyl)cytosine (4′-thioFAC), and several novel 4′-thionucleosides that possess antiherpes virus activities.