Jon A. Wolff, Robert W. Malone, and collaborators (University of Wisconsin-Madison / Vical Inc.) show that RNA and DNA expression vectors, injected separately into mouse skeletal muscle in vivo, produce protein expression comparable to that obtained in transfected fibroblasts in vitro under optimal conditions, with no need for any special delivery system (liposomes, viral vectors). It is the first demonstration that naked mRNA, injected directly, can be expressed in living tissue.