The pol I-based system was first used with influenza virus, whose replication occurs in the nucleus of infected cells (Neumann and Kawaoka, 2004) , but subsequently it has been used for the rescue of a variety of NS RNA viruses including bunyaviruses, filoviruses and arenaviruses (Flatz et al., 2006; Groseth et al., 2005; Habjan et al., 2008; Ogawa et al., 2007) .