1. Cell and Molecular Biology Laboratory, SRI International, 333 Ravenswood Avenue, Menlo Park, California 94025, Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Moscow 123182, Russia, Institute of Molecular Genetics, Russian Academy of Sciences, Moscow 123182, Russia, and Department of Pediatrics, Stanford University School of Medicine, Stanford, California 94305