Recombinant gp140 from HIV-1 isolates 92UG37, CA18, CN54, k530, UG21 and BR29 were provided by Simon Jeffs (Imperial College London, UK). Recombinant gp120 from HIV-1 isolates CM235, 93TH975, BaL and SF162 were obtained through the NIH-ARRRP, while gp160 from isolates MN and LAI were purchased by Prospec-Tany Technogene LTD (Israel). Recombinant YU2 gp120 and the two mutants D368R and I420R were provided by John R. Mascola and Richard Wyatt (VRC, NIAID, NIH, US). Recombinant soluble CD4 and recombinant gp120 from HIV-1 IIIB and CN54 were obtained from the CFAR, NIBSC (UK). The recombinant ectodomain of gp41 from isolate HxB2 (amino acids 541-682) was purchased by Vybion Inc. (US). HR-1 is a modified version of gp41 [46] exposing residues HLLQLTVWGIKQLQARILAVE (HxB2). 5HB was produced as described [47] . HR-1-FP comprises gp41-residues 512-594 (HxB2) including the fusion peptide and HR-1. HR-2 (HxB2) comprises gp41 residues 591-693 (HxB2) including the cys-loop region, HR-2 and MPER flanked by two coiled coil regions.