Correction to: Nature Methods https://doi.org/10.1038/nmeth.2255, published online 2 December 2012.

In the version of this article initially published, the recipients of grant RN2-00931 from the California Institute for Regenerative Medicine were listed as L.C.L., R.D.T., F.S.B. and J.F.L. The correct recipient is L.C.L. The error has not been corrected in the original article.