1. National Institute for Medical Research, The Ridgeway, NW7 1AA London, U.K., Centro Interdipartimentale per la Ricerca Biotecnologica, Universitá degli Studi di Bologna, Via Irnerio 42, Bologna, Italy, Universitá di Napoli, Via Cinthia, 80137 Napoli, Italy, and Institut de Génétique et de Biologie Moleculaire et Cellulaire, CNRS/INSERM/ULP, BP 10142, 1 rue Laurent Fries, 67404 Illkirch Cedex, France