Affiliation: | 1. Department of Molecular Biology and Microbiology, Tufts University School of Medicine, 150 Harrison Ave., Boston, MA 02111, USA.;2. Present address: Carnegie Institution, Baltimore, MD 21218, USA;3. Columbia University, New York, NY 10027, USA.;4. Howard Hughes Medical Institute, 150 Harrison Ave., Boston, MA 02111, USA. |