Moreover, neuroaminidases of these new H1N1 swine flu examples are more similar to H5N1 avian flu strains than other H1N1 variants found in the Americas or than that of the historic strains such as the 1918 Spanish flu [31] . This is surprising since avian flu strains typically have different hemaglutinin (HA) subtypes (e.g. H5N1). Combinations of HA and NA subtypes need to be fine-tuned to recognize the same type of sialic acid modifications to allow smooth interplay of the two proteins, which is important for the viral cycle [6] . These results support the notion that, also inside the family of N1 subtypes, a clear distinction can be made between avian-like H1N1, such as the one from the current outbreak, and other existing H1N1 strains.