1. Solomon H. Snyder Department of Neuroscience, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA.
2. Center for Sensory Biology, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA.
3. Biochemistry, Cellular and Molecular Biology Graduate Program, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA.
4. Howard Hughes Medical Institute and Department of Physiology and Biophysics, University of Washington, Seattle, WA 98195, USA.
5. Department of Biological and Environmental Sciences, University of Helsinki, Helsinki FI-00014, Finland.
6. Department of Ophthalmology, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA.