Phylogenetic analysis of the HA1 of H1N1 influenza viruses. Phylogenetic tree constructed using the neighbor-joining method and bootstrap analysis (n = 500) to determine the best-fitting tree for the HA1 region of the HA gene. Selected, relevant strains of the H1N1 subtype are listed by year of isolation. Small case "a" and "b" designate when two distinct strains co-circulated in the same year. Bootstrap values ≥ 70% are shown at branchpoints.