1. Institut de Biologie Moléculaire et Cellulaire, CNRS, INSERM, Université Louis Pasteur, 1 rue Laurent Fries, BP 163, 67404 Illkirch Cedex, C. U. de Strasbourg, France.
2. Division of Hematology/Oncology, Children's Hospital, Department of Medicine, Boston, MA 02115, USA.
3. Howard Hughes Medical Institute, Children's Hospital, Boston, MA 02115, USA.