To better characterize the epitope recognized by HGN194 we performed a replacement scanning of each position with the 18 complementary amino acids. This analysis revealed only 3 positions (RRSVRIGPGQTF) where amino acid substitutions abrogated binding. Remarkably, only one mutation out of the 21 found in viral isolates (I to M in position 6) affected binding of HGN194 ( Figure 4B-C) . However, we observed that several HIV-1 isolates that were not neutralized by HGN194 encoded the same amino acid sequence shared by other HIV-1 isolates that were neutralized ( Figure 4B ). For instance, the epitope RKSVRIGPGQTF on 93MW965.26, which is strongly neutralized by HGN194 (i.e. IC50 ,0.02 mg/ml), is shared with Du422.1, ZM197M.PB7 and Du172.17, isolates not neutralized by HGN194. This implies that the HGN194 V3 epitope is unavailable for antibody recognition on the assembled trimer of these non-neutralized isolates.