Growth and stability of rHP-PRRSV/SD16/TRS6-EGFP in Marc-145 cells. A. Marc-145 cells were infected with HP-PRRSV/SD16 or rHP-PRRSV/SD16/TRS6-EGFP and the amount of virus in the cultures was measured at various times from 12 to 96 h pi. B. rHP-PRRSV/SD16/TRS6-EGFP was passaged in Marc-145 cells every 3–4 days. The cells of each passage were freeze-thawed three times and used for virus titration. C. EGFP expression throughout virus passages in Marc-145 cells was analysed by quantitating relative fluorescence using a GFP Quantification Kit. Data were expressed as the mean ± standard deviation (SD).