Inhibitory activity of YIK-C16. (a) Inhibitory activities of HP23-E6-IDL, YIK, and YIK-C16 against cell-cell fusion between H9/IIIB cells and MT-2 cells; (b) Inhibitory activities of HP23-E6-IDL, YIK, and YIK-C16 against 6-HB formation between N46 and C34 peptides; (c–e) Inhibitory activities of HP23-E6-IDL, YIK, and YIK-C16 against HIV-1IIIB (X4 virus) infection when cells preincubated with peptides were washed to remove the unbound peptides, or not washed before addition of virus; (f–h) Inhibitory activities of HP23-E6-IDL, YIK, and YIK-C16 against HIV-1IIIB (X4 virus) infection when virus preincubated with peptides were washed to remove the unbound peptides, or not washed before addition of target cells.