Total RNA was extracted from the PRRSV CH-1a strain and the spleens of BALB/c mice. Full length cDNAs were amplified based on the complete open reading frames (ORFs) of M and ubiquitin (Ub) following reverse transcription (GenBank accession numbers AY032626 and AZ033428) using the M-F and M-R primers pair or the Ub-F and Ub-R primer pair ( Table 1) . The full length cDNAs were cloned into the pMD18-T vector (TaKaRa) and are referred to as pMD-M and pMD-U, respectively.