1. Department of Cellular and Developmental Biology, School of Medicine, University of Colorado Health Sciences Center, Denver, CO 80262
2. The Henry Wellcome Laboratory of Cell Biology, Division of Biochemistry and Molecular Biology, Faculty of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ, United Kingdom
3. Division of Gene Regulation and Expression, Wellcome Trust Biocentre, University of Dundee, Dundee DD1 5EH, United Kingdom
4. Department of Biochemistry, University of Leicester, Leicester LE1 7RH, United Kingdom
5. Genentech, South San Francisco, CA 94080-4990