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Phylogenetic trees of the HA, NA, NP, M, PB2, PB1, NS and PA genome segments of avian influenza A viruses isolated before 1981. The tk/ON/6213/66 and tk/ON/7732/66 viruses are shaded, and sequenced viruses from this study (Table 1) are indicated by ▪. Full trees and a list of GenBank accession numbers of the viruses used are provided in Fig. S1 and Table S1. Bars, nucleotide substitutions per site.
