This study was carried out in strict accordance with the recommendations in the Guide for the Care and Use of . Amino acidic sequence conservation in hemagglutinin groups and subtypes at the region bound by PN-SIA49. Circles below residues indicate PN-SIA49 percentage binding to each HA alanine mutant compared to binding to the wild-type HA: red 25% binding; yellow 50-75% binding, blue 100% binding. Sequence numbering is based on H1N1 A/Puerto Rico/8/1934 coding region (GenBank accession number ABO21709). Subtypes that can be neutralized by PN-SIA49 are indicated with a green '+' on the left, while the ones that can not be neutralized are indicate with a red '2'. a Recombinant HA from H1N1 A/South Carolina/1/1918 pandemic strain was previously shown to be bound by PN-SIA49 [26, 27] . b H1N1 A/New Caledonia/20/1999 was previously shown to be neutralized by PN-SIA49 as Fab fragment [26, 27] . doi:10.1371/journal.pone.0034415.g003