1. Howard Hughes Medical Institute, University of California at San Francisco, San Francisco, CA 94158, USA.
2. Department of Biochemistry and Biophysics, University of California at San Francisco, San Francisco, CA 94158, USA.
3. Departments of Pathology and Ophthalmology, University of California at San Francisco, San Francisco, CA 94158, USA.
4. Departments of Anatomy and Ophthalmology, University of California at San Francisco, San Francisco, CA 94158, USA.
5. Department of Cellular and Molecular Pharmacology, University of California at San Francisco, San Francisco, CA 94158, USA.