Twenty-one mAbs bound to gp41 by targeting primarily the immunodominant C-C region, the HR-1 and the region recognized by 5F3 mAb [64] (Table 2) . MAbs HGK129, HGN146 and HGN35 were assigned to the C-C loop since they competed with 3D6 [43] , reacted with an HR2 construct and recognized synthetic peptides within this region. MAbs HGW17 and HGY25 were provisionally assigned to the C-C region since they competed with 3D6. These data are consistent with the immunodominance of the C-C region and with the notion that the antibody response against this region overlaps with the 3D6 epitope. Several mAbs competed with 5F3 and HK20, and bound both the HR-1 and 5HB constructs, suggesting that they may bind between the fusion peptide and the HR-1 region. HGB33 was provisionally assigned to the HR-1 region according to competition with HK20 and binding to the 5HB construct. Other mAbs bound specifically the 5HB construct but did not compete with any of the mAbs tested, thus indicating that the complete HR-1 coiled coil region exposed in 5HB harbors antibody epitopes available for B cell recognition in the gp41 pre-hairpin conformation [65, 66] . The concomitant binding to recombinant gp41 suggests that the latter may partially resemble the gp41 prefusion state. Of note, three gp41 binding mAbs (HGP40, HGP48 and HGY50) neither competed with any of the mAbs tested, nor bound to any constructs representing pre-hairpin conformations or native gp140 ( Figure S1 ), indicating that they recognize as yet uncharacterized regions that are only available in the recombinant gp41 protein. Interestingly, mAb HGF24 was assigned by Pepscan analysis to an epitope in the C-terminal region of HR-2 (TNLIYTLIEESQN), proximal to the 2F5 epitope, and neutralized 4 Tier-2 isolates (Table S2) . This mAb competed with HK20 for gp41 binding, in spite of their distinct cognate specificities (HR-2 and HR-1, respectively). This finding would be consistent with the proximity of HR-1 and HR2 in the six-helix bundle structure. Finally, although the gp41 protein used in the primary screening includes the 2F5 and 4E10 epitopes, no MPER specific mAbs were isolated, reinforcing the idea that this portion of gp41 is poorly immunogenic in humans [21, 22] In conclusion, regardless of their limited breadth of neutralization, this extended panel of human mAbs may represent a useful tool for understanding the molecular basis of Env recognition in humans.