Bicistronic expression of gp55 proteins and GFP. Ba/F3 EpoR cells were infected with pMX-gp55-IRES-GFP retroviruses encoding gp55-A (B), gp55-APA (C), and gp55-P (D) cloned in the pMX-IRES-GFP retroviral vector or with a similar pMX retrovirus without an insert (A). The FACS analysis shows the percentage of cells that express different levels of GFP at 48 hours after infection. Cells with high GFP fluorescence (bars in B, C, and D) were sorted and then cultured in the presence of Epo. Greater than 78% of these cells retained high GFP expression, as indicated by the panels in the middle column depicting the sorted cells. These sorted cells were also assayed for Epo independence. Only cells infected with pMX-gp55-P-IRES-GFP virus could grow in the absence of Epo; FACS analysis of these is shown in (D), Selected. The level of expression of gp55 proteins in the respective cell lines was shown in the lower panel by a Western blot of whole cell lysates with the anti-gp55 antibody.