1. Department of Biophysics and Radiation Biology, Semmelweis University Faculty of Medicine, Puskin u. 9, Budapest H-1088, Hungary, Johnson Research Foundation, Department of Biochemistry and Biophysics, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-6059, and Department of Human Genetics and Molecular Medicine, Sackler Institute of Molecular Medicine, Sackler School of Medicine, Tel Aviv University, Ramat Aviv, P.O. Box 3904, Tel Aviv 69978, Israel