1. Howard Hughes Medical Institute, Baylor College of Medicine, Houston, TX 77030, U.S.A.
2. Department of Cell Biology, Baylor College of Medicine, Houston, TX 77030, U.S.A.
3. Department of Pediatrics, Baylor College of Medicine, Houston, TX 77030, U.S.A.
4. Institute for Molecular Genetics, Baylor College of Medicine, Houston, TX 77030, U.S.A.