1. T. N. M. Schumacher, L. M. Mayr, M. A. Milhollen, M. W. Burgess, P. S. Kim, Howard Hughes Medical Institute, Whitehead Institute for Biomedical Research, Department of Biology, Massachusetts Institute of Technology (MIT), Nine Cambridge Center, Cambridge MA 02142, USA.
2. D. L. Minor Jr., Howard Hughes Medical Institute, Whitehead Institute for Biomedical Research, Department of Chemistry, MIT, Nine Cambridge Center, Cambridge, MA 02142, USA.