[1] MRC, Cambridge Ctr Brain Repair, Cambridge CB2 2PY, England
[2] Univ Cambridge, Dept Anat, Cambridge CB2 3DY, England
[3] Univ Birmingham, Inst Child Hlth, Birmingham B16 8ET, W Midlands, England
[4] Sanger Ctr, Hinxton CB10 1SA, England
[5] Univ Calif San Francisco, Gladstone Mol Neurobiol Program, San Francisco, CA 94141 USA
[6] Univ Calif San Francisco, Dept Neurol, San Francisco, CA 94141 USA