We thank Stuart Bellamy, Lucy Catto, Mark Dillingham, Mark Szczelkun, and John Marko for aid and advice. This work was funded by Wellcome Trust Grant 063111/Z/00.
Author contributions: D.M.G. and S.E.H. designed research and analyzed data; D.M.G. performed research; G.G.W. contributed new reagents/analytic tools; and S.E.H. wrote the paper.
Conflict of interest statement: No conflicts declared.
This paper was submitted directly (Track II) to the PNAS office.