Parental (wild-type; WT) and 3T5T long terminal repeat (LTR) results in cell type–specific phenotypes within different promoter backbones. TF-1, U-937, and Jurkat cells were stably transfected with the HIV-1 LAI, YU-2, and 89.6 LTRs, which were placed in a green fluorescent protein expression vector (pEGFP-N1) using the AMAXA Nucleofector System (Lonza, Basel, Switzerland). Mutagenesis was used to introduce the position 3 C-to-T change at CCAAT/enhancer binding protein (C/EBP) site I and a position 5 C-to-T change at Sp site III (3T5T). Flow cytometric analysis was used to measure the ability of the variant LTR to drive GFP expression as compared with their parental (WT) counterparts. The fluorescence patterns in untransfected cells are depicted in gray. The LAI WT LTR is shown in red and the LAI 3T5T LTR is shown in blue.