1. Department of Molecular Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, Department of Biochemistry and Molecular Biology, University of Bergen, Årstadveien 19, N-5009 Bergen, Norway, and Center of Medical Genetics and Molecular Medicine, University of Bergen, Haukeland Hospital, N-5021 Bergen, Norway