Abstract
Statistical analysis of 21 nucleotide sequences of hemagglutinin 1 (HA1) gene of human influenza A viruses at antigenic and nonantigenic sites shows the involvement of positive selection at antigenic sites of the HA1 gene. Nucleotide diversities at nonantigenic sites are higher at synonymous sites than at nonsynonymous sites while at antigenic sites, nonsynonymous sites exhibit more nucleotide diversities.