Positive regulation of Pol III promoter activity using Dox-dependent transactivators bearing Sp1 transactivation domains. HOS-CD4-Fusin cells were co-transduced with a lentiviral vector expressing an shRNA corresponding to CXCR4 from the PittΔU6 promoter and lentiviral vectors encoding the rtTASp1(AB), rtTASp1(B), or rtTASp1(pB) transactivator, or the rtTA2s-M2 transactivator. Transduced cells were expanded in the presence of Dox for 7 days. Dox was then removed and cells were analyzed by flow cytometry at the times indicated. The results shown are representative of 3 similar experiments performed.