1. Department of Biochemistry and Molecular Biology, Medical College of Georgia, Augusta, GA 30912-2100, U.S.A.
2. Department of Physiology and Endocrinology, Medical College of Georgia, Augusta, GA 30912-2100, U.S.A.
3. Department of Genetics, Yale University School of Medicine, New Haven, CT 06510, U.S.A.
4. Department of Anatomy and Cell Biology and Program in Neuroscience, Emory University, Atlanta, GA 30322, U.S.A.