vector showed that the cHS4 element could effectively block transactivation of a nearby TATA-box minimal promoter in HeLa and primary human T cells. Incorporation of cHS4 sequences could therefore potentially lead to an increased safety profile. By following the expression of a silencing-prone promoter, we demonstrate in this study that the 
extent of silencing may depend on the type of carrier, most likely due to overall differences in the integration profile of the different DNA transposon carriers. We show that incorporation of cHS4 insulator sequences can lead to an increase in transgene expression levels for genomically integrated SB- and PB-based vectors in ARPE19 cells. In addition, improved stable transfection rates are obtained for cHS4-insulated SB vectors, possibly due to the increased mobilization of cHS4-containing transposons from plasmid DNA. Finally, we find that inclusion of cHS4 elements in SB-, PB- and Tol2-derived vectors does not lead to long-term protection against progressive transgene silencing in ARPE19 cells, supporting the notion that the barrier activity of the cHS4 insulator is not uniformly active in all cell types. DNA to obtain equal DNA amounts in each transfection. Transfections were carried out using FuGene-6 according to manufacturer’s Calicheamicin web instructions using 3 ml of reagent per 1 mg of DNA. One day after transfection, cells were split in varying densities and plated in 10-cm dishes. Two days after transfection, selection medium containing 1 mg/ml puromycin was added to the cells. After 8 days of selection, colonies of cells were stained with 0.6% methylene blue, air-dried and counted. Generation of stable expressing cell clones and longterm expression analysis ARPE-19 cells were seeded in 6-well dishes and transfected with 0.125 16699066 pmol transposon plasmid together with 0.05 mg transposase plasmid using FuGene-6 transfection reagent according to manufacturer’s instructions. One day after transfection cells were split in varying densities and plated in 10-cm dishes. Selection medium, containing 1 mg/ml puromycin, was added to the cells two days after transfection. After 10 days of selection, single clones were isolated and expanded for genomic DNA extraction and long-term eGFP expression analysis. The isolated, stably expressing cell clones were passaged for 8 weeks in standard culture medium, and analyzed by flow cytometry on day 0 and day 56 on a BD FACSAria III cell sorter. In the flow cytometric analysis, non-transfected cells were included as a negative control, and propidium iodide was used to exclude non-viable cells. Materials and Methods Plasmid construction The plasmids pSBT/RGIP, pSBT/cHS4.RGIP.cHS4, and pCMV-SB100X have been described previously. The pPBT/RGIP and pTol2T/RGIP plasmids were constructed by ligation of a RSV.eGFP.IRES.puro PCR fragment, amplified from pSBT/RGIP, into ClaI/NotI-digested pXL-BacII and NheI/ClaI-digested pT2AL200R150, respectively. To generate pPBT/cHS4.RGIP.cHS4 and pTol2T/cHS4.RGIP.cHS4, the 1200-bp cHS4 insulator element was amplified from pSBT/ cHS4.RGIP.cHS4 by PCR and inserted in front of and after the RSV.eGFP.IRES.puro cassette in pPBT/RGIP and in pTol2T/RGIP, respectively. The pCMV-PB and pCMV-Tol2 plasmids have been previously described in. The pCMV-SB100X.chloramp and pCMV-PB.chloramp plasmids were generated by ligation of a chloramphenicol PCR fragment amplified from 15647369 pBC SK+ into PvuI-digested pCMVSB100X and pCMV-PB, respectively. To generate pPBT4tp, the tran
