Nat. Genet.; doi:10.1038/ng.2627; published online 28 April 2013; corrected online 9 May 2013

In the version of this article initially published online, the affiliations of authors Xiuxiu Liu, Xiaogang Li, Nan Zhang and Na Liu were incorrect. The correct affiliation of these authors is the Department of Neurology, Peking University Third Hospital, Beijing, China. The errors have been corrected in the PDF, HTML and print versions of this article.