1. Howard Hughes Medical Institute and Laboratory for RNA Molecular Biology, The Rockefeller University, 1230 York Ave., Box 186, New York, NY 10065, USA.
2. Department of Obstetrics and Gynecology, Columbia University Medical Center, 630 W 168th St., New York, NY 10032, USA.
3. Department of Obstetrics and Gynecology, Albert Einstein College of Medicine, Bronx, New York, NY 10461, USA.