The sequences of the polymerase proteins (PA, PB1, and PB2) of the 1918 virus and subsequent human viruses differ by only 10 amino acids from the avian influenza viruses. Viruses with 7 of the 10 amino acids in the human influenza locations have already been identified in currently circulating H5N1. This has led some researchers to suggest that other mutations may surface and make the H5N1 virus capable of human-to-human transmission.