As the full-length M protein is difficult to express in vitro, we expressed a truncated version of M that lacks 84 hydrophobic amino acids at the N-terminus. SDS-PAGE analysis showed that cells transformed with the pGEX-M expression vector produced a large amount of protein with a molecular mass of approximately 36 KDa, consistent with the expected molecular weight of the truncated M protein fused with a GST tag (data not shown). Western blot analysis using an anti-PRRSV-M antibody confirmed the expression and identity of the truncated M protein fused with a GST tag (Additional file 1, Fig. S1 ).