1. Friedrich Miescher Institute for Biomedical Research, 4058 Basel, Switzerland
2. Current address: Hoffmann-La Roche Ltd. Pharmaceuticals Division, 4070 Basel, Switzerland
3. University of Basel, Faculty of Natural Sciences, 4056 Basel, Switzerland;
4. Current address: Department of Early Discovery Biochemistry, Genentech Research and Early Development, South San Francisco, California 94080