Thirteen amino acid residues used to classify NS1 sequences.

The most common amino acids (>1% frequency) in db4535 are listed on each position based on their frequency with the exception of A1, which is composed by three unique sequences only. Because of the large diversity in NS1, it is necessary to apply the whole set to classify sequences without applying phylogenetic methods. The number of sequences covered by the given residues is described as percentages. Those sequences with one or more polymorphisms away from this list are described as variants. A/Fort_Monmouth/1/1947 NS1 is the sequence with the largest number of differences (4) due to a truncation in amino acid 202. Residues in positions 25, 59, 217 and 227 were used to identify early A3 sequences collected between 1930 and 1942 are marked in italics. K26 and V90 are also characteristic of the A1 and A2 subtypes, respectively. The C group was excluded because of its large divergence from the A and B groups and the difficulties with achieving a satisfactory alignment.*Positions 215, 217 and 227 allow gaps in the A subgroups.