The CMV and chicken β-actin promoters cannot drive gene expression in bicistronic vectors transfected into CCE cells. CCE cells (A) or rat embryo fibroblasts (B) were transfected with the bicistronic vectors pIRES2-EGFP and pIRES-EGFP(β-actin) in which the CMV promoter had been substituted with the chicken β-actin promoter. GFP expression was not evident in CCE cells transfected with these vectors; GFP+ cells were only obtained when cells were transfected with pEGFP-N1. In contrast GFP expression was apparent in fibroblasts transfected with all three constructs (B). Micrographs were obtained 24 hours after transfection.