1. Department of Molecular Biology, University of Texas Southwestern Medical Center, Dallas, TX 75390–9148, USA.
2. Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, TX 75390–9148, USA.
3. Howard Hughes Medical Institute, University of Texas Southwestern Medical Center, Dallas, TX 75390–9050, USA.
4. Department of Biochemistry, School of Medicine, Vanderbilt University, Nashville, TN 37235, USA.
5. Department of Biomedical Engineering, St. Louis University, St. Louis, MO 63103–2010, USA.