Presentation of PB1-F2 variants of mammalian and avian influenza A viruses. Nine PB1 lineages from A to I have been described and some selected sequences within these lineages are presented. Lines at the top of the Figure represent amino acids of the predicted helical region (black) and the putative mitochondrial targeting sequence (dashed). Amino acids that are considered to enhance viral pathogenesis are marked in grey. The first stop codon has been shown by asterisk and following stop codons are indicated by subsequent asterisk.