Schematic representation of the recombinant pH1N1 viruses. (A) PA (left) and PA-X (right) WT (gray) and MUT (black) viral proteins. The 5 amino acid changes in pH1N1 PA (V100I, P224S, N321K, I330V, and R362K) and the 4 amino acid changes in pH1N1 PA-X (V100I, N204S, R221Q, and L229S) selected in currently circulating pH1N1 viruses are indicated. Numbers on the top indicate the amino acid lengths of the PA and PA-X proteins. The PA and PA-X proteins share the N-terminal 191 amino acids. The +1 frameshift is indicated with a striped bar. (B) pH1N1 NS1 WT (gray) (top) and MUT (black) (bottom) proteins containing 6 amino acid differences (E55K, L90I, I123V, E125D, K131E, and N205S) in currently circulating strains (38) are indicated. The number on the top indicates the amino acid length of WT and MUT NS1 proteins.