1. Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139, USA.
2. Department of Biochemistry, University of Wisconsin-Madison, 433 Babcock Drive, Madison, WI 53706-1544, USA.
3. Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, WI 53706-1322, USA.
4. Complex Carbohydrate Research Center, 315 Riverbend Rd, Athens, GA 30602, USA.
5. Institute of Biology, University of Leiden, 2333 BE Leiden, Netherlands.
6. The Broad Institute of MIT and Harvard, Cambridge, MA 02142, USA.