A 3-nucleotide mutation diminishes the shRNA mediated CCR5 reduction. PHA/IL-2 activated PBMCs were transduced with a lentiviral vector bearing either shRNA 1005 [CCR5 shRNA (1005)] or the mutant containing 3-nucleotide substitution [mutant (1005)]. To monitor the expression levels of CCR5 on cell surface, the cells were cultured for 12 days and then stained with APC conjugated anti-CCR5 monoclonal antibody. CCR5 and EGFP expression were monitored by flow cytometry. The percentage number in each quadrant is indicated in each panel.