HJ16 and b12 neutralized 1/10 and 8/10 Tier-1 and 32/82 and 35/82 Tier-2 pseudoviruses, respectively ( Figure 2) . Interestingly, 22/32 Tier-2 isolates neutralized by HJ16 were not neutralized by b12, and reciprocally 24/35 Tier-2 isolates neutralized by b12 were not neutralized by HJ16. Another interesting finding is that HJ16 does not discriminate between clades as much as b12, 2G12 and 2F5 (e.g. b12 and 2G12 rarely neutralize clade A isolates while 2F5 and 2G12 rarely neutralize clade C isolates, Table S3 ). These results reveal a largely nonoverlapping pattern of reactivity of HJ16 and b12 and suggest that the combination of these two mAbs could be effective against a dominant fraction of HIV-1 isolates (57/82, i.e. 69%). Taken together these results are consistent with HJ16 recognition of a surface proximal to the CD4bs domain within gp120, but with an epitope completely distinct from that recognized by b12.