Mutation at position 622 in the PB1 protein affects the polymerase activity and replication of H5N1 viruses in vitro. (A) Polymerase activities of the DK/08 and CK/09 RNP complexes with different PB1 mutations in a minigenome assay. The values shown are means ± standard deviations of results for three independent experiments and are standardized to the activity of CK/09 (100%). *, P < 0.01 (compared with the results seen with CK/09 RNP-transfected cells); **, P < 0.01 (compared with the results seen with DK/08 RNP-transfected cells). WT, wild type. (B) Multicycle replication of H5N1 avian influenza viruses in MDCK cells. MDCK monolayers were inoculated at an MOI of 0.001 with virus, and the culture supernatants were collected at the indicated time points and then titrated in MDCK cells. *, P < 0.01 (compared with titers in CK/09 virus-infected cells); **, P < 0.01 (compared with titers in DK/08 virus-infected cells). The Student-Newman-Keuls test was used for the statistical analysis.