Presence of homologous virus M1 and NP increases vRNP content in virions. MDCK cells infected with WSN, WSN-AichiM1, or WSN-AichiM1/NP virus at an MOI of 1 were labeled with [35S]methionine-cysteine for 24 h, and purified virions were analyzed by SDS-PAGE. Expression of NP and M1 in cell lysates was examined by immunoprecipitation assay. Intensities of the protein bands were quantitated, and relative ratios are shown. RIP, radioimmunoprecipitation.