HIV-1–based SIN lentiviral vectors. There were 3 sets of vectors constructed containing (A) the MU3, (B) the EF1α, or (C) the CAG promoter driving expression of the enhanced GFP gene as reporter. Each set of vectors includes a control vector, a 5′HS4 element–containing vector, an IFN-SAR–containing vector, and a 5′HS4/IFN-SAR–containing vector. A 1.2-kb fragment containing the 5′HS4 insulator was inserted in direct orientation into the deleted U3 region (ΔU3) of the 3′ LTR, which is copied to the 5′ LTR following reverse transcription. The 0.8-kb IFN-SAR was introduced in reverse orientation into the 3′ untranslated region of the GFP gene, immediately upstream of the 3′ LTR. The arrows above the 5′ LTR and the internal promoters depict sites and direction of transcription. WPRE indicates woodchuck hepatitis virus posttranscriptional regulatory element; SAR, IFN-SAR; SD, splice donor; SA, splice acceptor; ΔGag, deleted Gag region; and RRE, Rev-responsive element.