In vitro selection of escape mutants. To further define the region bound by PN-SIA28, an assay aiming at the evaluation of PN-SIA28 capability to induce escape mutant was performed. A/ PR/8/34 H1N1 virus was cultured under constant selective pressure of PN-SIA28, and after several rounds of cell infections, two escape mutants were generated. Sequencing analysis of the generated neutralization escape variants revealed two different mutants carrying each a single amino acidic mutation in position 361 (Ile361Thr) and in position 362 (Asp362Gly) in the HA2 subunit compared to wild type.