1. Howard Hughes Medical Institute, University of California San Francisco, San Francisco, CA 94143, USA.
2. Department of Pediatrics, University of California San Francisco, San Francisco, CA 94143, USA.
3. Eli and Edythe Broad Center of Regeneration Medicine and Stem Cell Research, University of California San Francisco, San Francisco, CA 94143, USA.
4. Wolfson Institute for Biomedical Research and Research Department of Cell and Developmental Biology, University College London, London WC1E 6BT, UK.
5. Department of Neurosurgery, University of California San Francisco, San Francisco, CA 94143, USA.
6. Department of Psychiatry/Langley-Porter Institute, University of California San Francisco, San Francisco, CA 94143, USA.