1. Department of Physiology, Semmelweis University, P.O. Box 259, H-1444 Budapest, Hungary, Laboratoire DBMS/BBSI, CEA Grenoble, 17 Rue des Martyrs, 38054 Grenoble Cedex 9, France, and Massachusetts General Hospital Cancer Center and Harvard Medical School, Building 149, 13th Street, Charlestown, Massachusetts 03125