S.Z. Ben-Sasson1,3, K. Wang2, J. Cohen2 and W.E. Paul1 1Laboratory of Immunology, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland 20892-1892 2Laboratory of Infectious Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland 20892-1892 3Lautenberg Center for General and Tumor Immunology, The Hebrew University/Hadassah Medical Center, Jerusalem 91120, Israel Correspondence: wpaul{at}niaid.nih.gov