1. Department of Microbiology and Immunology, Stanford University School of Medicine, Stanford, CA 94305, USA.
2. Department of Program in Immunology, Stanford University School of Medicine, Stanford, CA 94305, USA.
3. Unité du Développement des Lymphocytes, Centre National de la Recherche Scientifique, Unité de Recherche Associée 1961, Institut Pasteur, 75724 Paris, France.