Taken together these results indicate that HGN194 has unusual potency and breadth for a V3-specific monoclonal antibody. Of note, HGN194 appears to have a broader reactivity than 447-52D since it neutralizes all Tier-1 isolates and 11% of Tier-2 isolates, while 447-52D neutralizes 88% of Tier-1 and 4% of Tier-2 isolates (Figure 2 and Table S3 ). Another V3 mAb with unusual breadth of activity is F425-B4e8 [37] . We have not compared HGN194 with F425 here, but F425-B4e8 appears somewhat broader than 447-52D, neutralizing 1 clade C and 2 clade D pseudoviruses [37] . Further comparison between F425-B4e8 and HGN194 is difficult as different viruses and assays were used to determine neutralization breadth and potency. HK20, an HR-1 Specific mAb with Target Cell-Specific Neutralizing Activity HK20 was initially characterized as a gp41-specific mAb with broad neutralizing activity in the HOS-based assay but lacking robust activity in the TZM-bl assay. To map the epitope, we tested HK20 against all overlapping 15-mer peptides of the extracellular region of HIV-1 gp41. HK20 bound peptide QQHLLQLTVWGIKQL that overlaps the hydrophobic pocket sequence of HR-1 ( Figure 5A ). The specificity of HK20 was confirmed by immunoprecipitation of the 5-helix bundle (5HB) construct [47] and of a trimeric HR-1 construct that includes the gp41 fusion peptide, indicating that the fusion peptide does not interfere with HK20 binding ( Figure 5B ). In ELISA, HK20 bound to 5HB and HR-1 gp41 constructs with K 50 values of 210 and 95 ng/ml, respectively and also to the gp41 ectodomain, although with lower avidity (1.27 mg/ml) ( Figure 5C ), a finding that may be due to the partial unfolding of solid phase-bound gp41. Taken together, the above results indicate that HK20 recognizes a highly conserved site within the HR-1 region of gp41.