Relative susceptibility of NMBR-deficient cells to PR8 infection. The sh-NMBR cells and sh-Luciferase cells were infected with PR8 (MOI = 1) and collected at 0, 6, and 12 hpi. The expression of NP was assessed by RT-PCR (A), qRT-PCR (B) and WB (C). Culture supernatants were collected to test the titres by plaque assay at 12, 24, 36, 48 and 64 hpi (D). GAPDH was used as the reference housekeeping gene for internal standardization. **P < 0.01.