The p2luc-BM2 plasmid series was prepared by subcloning sequences encompassing the influenza B virus terminationreinitiation signal (prepared by RT-PCR) into the dual-luciferase reporter plasmid p2luc [21] . Most of these plasmids have been described previously [11] . The p2luc-BM218S-AG, 18S-AT and 18S-AC plasmids were generated by site directed mutagenesis of p2luc-BM2wt or p2luc-BM2-204 (see below).