1. Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 06520-8114, Department of Pharmacology, Yale University School of Medicine, New Haven, Connecticut 06520-8066, and Departamento de Genética Molecular, Instituto de Fisiología Celular, Universidad Nacional Autónoma de México, Ap. Postal 70-243, México D.F. 04510, México