Increase in gene expression mediated by the PRE depends on length of the fragment used. (a) SW480 cells were transduced with retroviral vectors (MP110-based) containing different PRE fragments. A 48 h post-transduction total RNA was prepared as previously described4 and analyzed by Northern blot using a 3′ UTR probe indicated in Figure 1b. The blot was reprobed with a GAPDH-specific probe as a loading control. Molecular weight standard is given on the left. Note that transduction efficiencies of cells transduced with the indicated vectors varied as stated in Table 1. (b) To determine the enhancement on gene expression at the protein level, SW480 cells were transduced with retroviral vectors harboring different PRE fragments as indicated and described in Figure 2a and analyzed by FACS. The mean fluorescence intensity (MFI) is given on the left.