Screening of a random 12-mer peptide phage displayed library. In order to select peptides able to bind antibody PN-SIA28 that could possibly indicate the region that the antibody recognizes on the HA, a phagemidic random 12-mer peptide library was screened. After three rounds of panning, PN-SIA28 binding phages were eluted and used to infect E. coli XL1-Blue for an ELISA screening on PN-SIA28. Twenty-five positive clones were sequenced. An in silico analysis was performed, using Mimox and Pepitope servers, comparing the selected peptides to the available crystal structure of H1N1 A/South Carolina/1918 (A/ SC/1918) (ID code 1RD8) and A/Puerto Rico/8/34 (A/PR/8/ 34) hemagglutinins (ID code 1RU7). This study allowed the identification of several possible residues located on the stem region of HA potentially involved in the binding of PN-SIA28: Asn336, Ile337, Pro338, Trp357 and Thr358 (sequence numbering refers to A/PR/8/34, GenBank accession number ABO21709) ( Figure 2 ).