Phylogenetic tree based on nucleotide sequence of the entire hemagglutinin gene of four H1N1 viruses detected in Study 2 and relevant standards representing different clusters of H1 viruses [a = alpha, b = beta, d1 = delta1, g = gamma, and p = pandemic (pdm(H1N1)]. The tree is built using Neighbor-Joining method and its reliability is estimated using 1000 bootstraps. Note that standard names have been shortened and modified so that the end of the name after underscore represent the cluster designation (a–p). The study viruses are labeled with the pig identification number, followed by sampling number. The scale represents number of substitutions per site.