Thirty-seven mAbs bound to gp120 targeting primarily the CD4bs and the V3 loop and to a lesser extent the CD4i site ( Table 1) . A first group of mAbs was assigned to the V3 loop based on cross-competition with two V3 loop specific mAbs (HR10 and HGA9). This group comprised 8 mAbs, which were already mapped to the V3 loop using the Pepscan analysis approach, and 7 additional mAbs that were not analyzed by peptide scanning (i.e. HGF9, HGT4, HGP21, HGP51, HGW48, HGD129 and HGP27). Four mAbs (HX44, HGP105, HGW7, HGY38) reacted with wt and CD4bs YU2 mutants but failed to bind the CD4i mutant, and therefore were assigned to the CD4i cluster. Notably, one mAb (HGP68) was mapped by Pepscan analysis to a novel epitope in the V2 loop (TVYALFYRLDIVP) and neutralized 4 Tier-1 isolates (Table S2) . Fifteen other mAbs were assigned tentatively or conclusively to the CD4bs based on their capacity to inhibit gp120-sCD4 binding. Most of these mAbs competed other CD4bs-specific mAbs, such as b12 and HJ16, while in some cases this assay could not be performed due to lack of epitope expression on the gp120 proteins used. Furthermore, this group of mAbs showed variable reactivities with the YU2 mutants. While most of them did not bind the D368R mutant similar to b12, others including HJ16, bound this mutant avidly. In addition, most of these new CD4bs-specific mAbs also showed decreased binding to the CD4i mutant, suggesting that they may span a broader, as yet undescribed region that includes elements of the CD4bs and CD4i sites. Finally, for 2 mAbs these analyses did not provide any relevant information.