1. Verna and Marrs McLean Department of Biochemistry,
2. Department of Molecular and Human Genetics,
3. Howard Hughes Medical Institute, Baylor College of Medicine, Houston, TX 77030, USA.
4. Howard Hughes Medical Institute and
5. Program in Molecular and Cell Biology, Oklahoma Medical Research Foundation, Oklahoma City, OK 73104, USA.
6. Department of Biochemistry and Molecular Biology, University of Oklahoma Health Sciences Center, Oklahoma City, OK 73190, USA.