Schematic diagram of rHP-PRRSVs containing EGFP gene under the control of TRSs and detection of EGFP expression by fluorescence microscopy. A EGFP gene was under the control of the different structural genes TRSs, located upstream of the start codon of each structural genes and varied in length and sequence. Each transcriptional unit containing the EGFP gene and six different length TRSs of HP-PRRSV was inserted between the N protein and 3′-UTR in the HP-PRRSV genome. B Marc-145 cells infected with six different recombinant HP-PRRSVs expressing the EGFP gene were observed for CPE and fluorescence detection. Live cells were analysed by phase contrast and fluorescence microscopy.