1. Committee on Higher Degrees in Biophysics, Harvard University, and Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, 240 Longwood Avenue, Boston, Massachusetts 02115, Vertex Pharmaceuticals Inc., 40 Allston Street, Cambridge, Massachusetts 02139, and Groupe de Cancérogenèse et de Mutagenèse Moléculaire et Structurale, ESBS, Bd Sébastien Brant, 67400 Strasbourg-Illkirch Cedex, France