Frédéric Martinon and collaborators (INSERM, Paris; in collaboration with Pierre Meulien's group at Pasteur Mérieux Sérums et Vaccins) immunize mice with liposomes containing messenger RNA encoding the influenza virus nucleoprotein. They demonstrate in vivo induction of virus-specific cytotoxic T lymphocytes (CTL), with specificity equivalent to that induced by the actual infectious virus. It is the first experimental demonstration of the mRNA vaccine concept.