Evidence for distinct contributions of heavy and light chains to the restriction of antibody recognition of the human immunodeficiency virus type-1 principal neutralization determinant : Brynmor A. Watkins, Alberta E. Davis, Simona Fiorentini, Fulvia Veronese, Marvin S. Reitz, Jr., Laboratory of Tumor Cell Biology, National Cancer Institute, National Institutes of Health, Bethesda MD 20892-4255, USA