Some investigators have used thymidine kinase (TK) deletion alone, whereas others have used viruses with dual deletion of viral genes encoding both TK and the viral growth factor B18R– or dual deletion of viral genes encoding TK– and viral growth factor (VGF).62 These genetic modifications aim to enhance tumor selectivity while retaining potency when infecting tumor but not normal cells.15,30. The gene discussed is TKT; the disease is neoplasm.