On page 2574 in the 28 March 2013 issue, the sixth author’s name, shown as Jeffrey Pollard, is incorrect. The correct name is Jeffrey W. Pollard.

The Authorship Contribution on page 2577 should have read: “J.A.S. and R.A.L. designed the experiments and wrote the manuscript. J.A.S., S.R., and K.B. performed the experiments. A.C.A. and R.F.N. provided technical assistance and advice. J.W.P. and N.F. developed critical reagents.” The errors have been corrected in the online version, which now differs from the print version.