The improved surface-complementarity may provide a structural explanation for the previous finding that the para-chloro- and meta-fluoro-substituted aromatic ring enhances JRC-II-191 binding affinity for YU2 gp120 5-fold and JRC-II-191 inhibits HIV-1YU2 infection of cells expressing CD4 and CCR5 more potently than NBD-556 does [21], [27]. This evidence concerns the gene CD4 and infection.