Test of U6 promoters in HeLa G/R cells. Changes in relative EGFP expression levels in HeLa G/R cells determined by flow cytometry using lentiviral vectors encoding an shRNA corresponding to EGFP controlled by various U6 promoters. For transduction, cells were exposed to 50 MOI (HOS cell units determined by real-time PCR). Expression levels are represented by MFI values of transduced cells compared to mock transduced cells, which are referred to as percentage of control MFI. Results shown were taken 7 days posttransduction and represent the mean ± SD of at least three experiments. hU6, human U6 promoter; Tet-hU6, human U6 promoter with an internal TO; mU6, mouse U6 promoter.