Evolutionary relationship of PB2 and PB1 genes of Danish HP H5N1 AIVs compared to other past and present H5N1 viruses. The nucleotide coding region trees were generated by maximum parsimony with heuristic random branch swapping search (neighbor joining and maximum likelihood analyses revealed similar tree topologies). Bootstrap values of 1000 resamplings in per cent (>70%) are indicated at key nodes. The clade with Danish isolates is marked in yellow. Figure 2 a) and b) demonstrate the evolutionary relationship of the PB2 and PB1 genes, respectively. Trees are midpoint rooted for means of clarity.