1. Department of Neurobiology, University of California, San Diego, La Jolla, CA 92093
2. Sanford Consortium for Regenerative Medicine, La Jolla, CA 92037
3. Department of Pathology, University of California, San Diego, La Jolla, CA 92161
4. Institut NeuroMyoGène, CNRS UMR5310, INSERM U1217, Faculté de Médecine Rockefeller, Université Claude Bernard Lyon I, 69008 Lyon Cedex, France