1. Department of Immunology, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA.
2. Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA.
3. Department of Molecular Biology, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA.
4. Howard Hughes Medical Institute, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA.