University of Cambridge - Wellcome Trust and MRC Cambridge Stem Cell Institute, University of Cambridge - Wellcome Trust and MRC Cambridge Stem Cell Institute, University of Cambridge - Cancer Research UK Cambridge Institute, Wellcome Sanger Institute, affiliation not provided to SSRN, University of Cambridge - Wellcome Trust and MRC Cambridge Stem Cell Institute, University of Cambridge - Wellcome Trust and MRC Cambridge Stem Cell Institute, affiliation not provided to SSRN, University of Cambridge - Wellcome Trust and MRC Cambridge Stem Cell Institute, University of Cambridge - Wellcome Trust and MRC Cambridge Stem Cell Institute, University of Cambridge - Wellcome Trust and MRC Cambridge Stem Cell Institute, University of Cambridge, Government of the United Kingdom - Manchester Centre for Clinical Neurosciences, Wellcome Sanger Institute, University of Cambridge - Academic Division of Neurosurgery, University of Cambridge - Division of Academic Neurosurgery, University of Cambridge - Division of Academic Neurosurgery, University of Birmingham, University of Cambridge - Division of Academic Neurosurgery, University of Cambridge, University of Cambridge - Academic Division of Neurosurgery, University of Cambridge, University of Edinburgh - Centre for Regenerative Medicine (Institute for Stem Cell Research), University of Cambridge - Wellcome Trust and MRC Cambridge Stem Cell Institute, University of Cambridge, affiliation not provided to SSRN, Wellcome Sanger Institute, University of Oxford and University of Cambridge - Wellcome Trust and MRC Cambridge Stem Cell Institute