To determine if the peptides interefere with virus:cell interactions, we conducted two different experiments. We first performed Design of Entry Inhibitor Peptides www.plosntds.org hemagglutination inhibition assays, but were unable to detect any inhibition of the ability of viral antigen to agglutinate red blood cells (data not shown). To further investigate virus:cell binding in a more relevant system, we treated virus with DN57opt or 1OAN1, bound the virus to cells, and washed the cells repeatedly at 4uC before measuring the amount of virus remaining on the cells by quantitative rt-PCR. Both peptides showed evidence of ability to block virus:cell binding compared to control virus without peptide (Figure 7) . Treatment of virus with pooled human anti-dengue serum or heparan sulfate similarly showed reduced cell binding.