In PLoS Biology, volume 4, issue 11: DOI: 10.1371/journal.pbio.0040348
The author contributions were listed incorrectly. The correct author contributions are as follows: BAH conceived and designed the experiments. MS, ML, JNN, LGF, CMC, and BAH performed the experiments and analyzed the data. MS, LGF, JNN, and BAH wrote the paper.
