Viral infection is a quite simple approach for stably introducing transgenes (e.g., those encoding short-hairpin RNAs [shRNAs]) into the genome. The critical aspects are that the virus and the target cell should be appropriately matched. For example, a virus bearing an ecotropic envelope protein will not infect a human cell line unless the appropriate receptor has been purposefully expressed. VSV-G (vesicular stomatitus virus glycoprotein) pseudotyped viruses have the greatest host range. Nondividing cells can only be infected with lentiviruses, but the additional safety precautions necessary for the use of these tools should dissuade their application to routinely cultured cell lines.