Comparison of the NA protein amino acid sequences among influenza A and B viruses. Multiple alignment was performed by CLUSTALW (available at http://www.genome.jp/tools/clustalw/). Protein accession numbers are shown in brackets next to influenza virus strain names. The conserved peptide studied by Gravel et al.  is underlined, and amino acid residues considered to be involved in the enzymatic activity  are shaded.