For consistency with previous studies of A-H3N2 HA evolution, we use the HKY model of nucleotide substitution [10] . We use the TREBLE algorithm, which implements a MCA, on sets of sequences solely drawn from a single subtype to derive within-subtype rates. The phylogenetic tree, generated by TREBLE, for the HA gene is depicted in Figure 2 (± 0.48) × 10 -3 s/s/yr and the subtype C rate is 1.31 (± 0.33) × 10 -3 s/s/yr. Lastly, for the NS gene, the rates are similar to those of the MP gene. The subtype A-H3N2 rate is 2.14 (± 0.25) × 10 -3 s/s/yr, the subtype B rate is 1.92 (± 0.20) × 10 -3 s/s/yr, and the subtype C rate is 1.68 (± 0.51) × 10 -3 s/s/yr. Table 1 presents these results. Figure 3 provides histograms of the bootstrap distributions for all three genes and subtypes.