A nucleoside analog; inhibits replications of HSV-1 KOS strain (ID50 = 0.2 μg/mL); reduces virus-induced cytopathogenicity of HSV-1, HSV-2, and vaccinia virus strains in PRK cells (MICs = 0.2-0.4, 1-2, and 5 μg/ml, respectively); selectively inhibits thymidylate synthetase (ID50s = 3 and 120 μg/ml for incorporation of [1',2'-3H]deoxyuridine and [CH3-3H]deoxythymidine, respectively); has been used to monitor DNA synthesis and cellular replication via click chemistry conjugation to fluorochromes,