The anti-influenza activity conferred by both V5-ssMx1 isoforms was evaluated in vitro using either the transfection of 3T3 cells followed by plaque assays [42] or transfection of Vero cells followed by the flow cytometric determination of the fraction of influenza virus-infected cells among Mx1-producing and nonproducing cell populations [43]. This evidence concerns the gene MX1 and influenza.