The expression of M was further confirmed by IFA. Briefly, BHK-21 cells were infected with either rWR-PRRSV-M or the vaccinia virus WR strain when the BHK-21 cells reached 70-80% confluence in a 24-well plate. M protein expression was evaluated by IFA 3 days post infection using the anti-PRRSV-M monoclonal antibody followed by a FITC-conjugated rabbit antimouse IgG secondary antibody (Sigma). Specific fluorescence was observed using a fluorescence microscope (LEICA DM IRE2).