Regulated knockdown of CXCR4 chemokine receptor expression in HOS-CD4-Fusin cells. Knockdown of cell surface CXCR4 expression was determined by flow cytometry following transduction of HOS-CD4-Fusin cells with lentiviral vectors expressing Tet-tTS, controlled by the CMV promoter and/or a CXCR4 shRNA, controlled by hU6 promoters. Cells were incubated in the presence or absence of Dox for 6 days. CXCR4 receptor levels were determined by flow cytometry after incubation of the cells with PE-labeled anti-human CXCR4 antibody. The results shown represent the mean ± SD of 3 experiments. They are displayed as the percentage of the MFI of mock-transduced cells.