A new strain of swine influenza A called H1N1 was responsible for the swine flu outbreak in 2009 in humans. It came from a combination of several strains of influenza in humans and influenza in birds and swine influenza. Tests were done. Testers found that the hemagglutinin (HA) gene was similar to that of swine flu viruses that had already been found in pigs in the United States since 1999, but the neuraminidase (NA) and matrix protein (M) genes resembled versions in European swine. Viruses that were built like this had not been found in humans or pigs before. There is no program to monitor what viruses are circulating in pigs in the United States, however it began in Mexico, North America.