1. Department of Biochemistry & Biophysics, Center for RNA Biology, and Center for AIDS Research, University of Rochester School of Medicine & Dentistry, Rochester, NY 14642, USA
2. Department of Chemistry, Colorado State University, Fort Collins, CO 80523, USA
3. Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
4. Koch Institute, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
5. Broad Institute of Harvard and MIT, Cambridge, MA 02139, USA
6. Department of Biochemistry & Molecular Biology, Colorado State University, Fort Collins, CO 80523, USA