MST312 is a telomerase inhibitor (IC50 = 0.67 μM in a TRAP assay). It is selective for telomerase over DNA polymerase when used at concentrations up to 3 μM. MST312 inhibits growth of U937 cells via telomere shortening (GI50 = 1.7 μM). It reduces cell number and expression of the proliferation marker MIB-1 as well as increases DNA damage in primary pediatric ependymoma tumor cells. MST312 decreases survival of H460 and H1299 non-small cell lung cancer (NSCLC) cells in a dose-dependent manner with a greater effect on the aldehyde dehydrogenase positive (ALDH+) cancer stem cell population. In vivo, MST312 (40 mg/kg) reduces tumor size by 70% in an H460 mouse xenograft model. MST312 also inhibits replication of herpes simplex virus 2 (HSV-2) and a clinical isolate of HSV-1 and decreases accumulation of early and late viral proteins in HEp-2 cells infected with HSV-1.