Viral titers of recombinant viruses containing amino acid changes in H5N1 NS1 and PA proteins: Groups of 7-to-8-week-old A129 male mice (N = 8/group) were infected with 1000 (103; A) or 10,000 (104; B) FFU/mouse of the indicated viruses (PAOR/NS1OR, PAOR/NS1CIR, PACIR/NS1OR, and PACIR/NS1CIR). Mice were sacrificed at 2 (N = 4) and 4 (N = 4) d p.i, and right lungs were harvested, homogenized, and used to quantify viral titers by immunofocus assay (FFU/mL. *, p < 0.05 (PAOR/NS1OR vs. PAOR/NS1CIR, PACIR/NS1OR, and PACIR/NS1CIR) using 2-way ANOVA (N = 4 per time point). Non-significant when p > 0.05.