Transfection efficiencies and expression levels in transiently transfected HEK293 and NIH3T3 cells. Transient transfection efficiencies (A, B) and the transient mean EGFP expression levels per cell (C, D) obtained by transfection of the vectors #1-8 into HEK293 cells (A, C), or NIH3T3 cells (B, D). All vectors contain the EGFP-IRES-BSD transgene transcription unit. Mean values of n = 8 derived from four independent experiments ± SD are shown; * p < 0.05; ** p < 0.001 (two-tailed Student's t-test). mock: untransfected control. Grey bars indicate vectors containing an hCMV/EF1P promoter element, black bars indicate vectors containing a CMV-IEP promoter element.