Comparison of the viral replication in mice of different 2009/H1N1 strains in a dose dependent manner. Five mice of each group were infected with a dose series of the viruses (102-105 TCID50), euthanized at 3 and 6 dpi and the titers in lung homogenates of mice were determined using absolute quantitative PCR on 3 (A) and 6 dpi (B) respectively. The values were calculated through a standard curve which was generated by ten-fold serial dilutions of standard plasmid harboring NP gene of 2009/H1N1 virus. The results are presented as mean ± SEM of virus copy numbers. × indicates the infection dose in the groups exceeds the content of the original virus. ND means the virus could be not detected. Significant differences were observed at 3/10 and F9/10 compared with 5/10, 6/10 and LN/09 (* p < 0.05).