Outline of lentiviral vectors. Schematic drawing of vectors used in this study. Vectors A-D utilize the CMV-promoter whereas vectors E-F use the EF1α-promoter. (A, E) Control vectors for the group of CMV and EF1α vectors respectively. Cloning sites for the cis-elements are shown. (B) One tandem repeat (2 × 250) of the core element of chicken β-globin insulator (cHS4) was inserted (sense orientation) prior to the promoter. (C, F) Two copies of the tandem repeat were inserted into the vectors – one just upstream the promoter and one just downstream the expression cassette. (D) The entire cHS4-insulator (1.2 kb) was inserted in the 3' LTR in sense orientation. (A-F) Horizontal lines denote the location of primers (labelled LV2, GFP, WPRE and INS) used for quantitative real-time PCR (qPCR). Legend: LTR – Long Terminal Repeat, ψ – packaging signal, RRE – Rev Responsive Element, cPPT – Central Poly Purine Tract, CMV – Cytomegalo virus promoter, EF1α – Elongation Factor 1α promoter, GFP – Green Fluorescent Protein, WPRE – Woodchuck Posttranscriptional Regulatory Element, 2 × 250 – tandem repeat of the cHS4 core element and 1.2 kb – full length cHS4 insulator sequence.