Bicistronic viruses encoding gp55 proteins and GFP.The pMX retroviral vector was modified to include the encephalomyocarditis virus IRES and, downstream, the GFP coding sequence. The cDNAs coding for gp55-P, gp55-APA, and gp55-A were inserted upstream of the IRES generating pMX-gp55-IRES-GFP vectors.