incorporation of cHS4 sequences into the SB and PB vectors could lead to increased gene expression levels from the transgene cassette. Similarly, insulated SB and PB clones containing 1 to 3 transposon insertions exhibited between 1.4 and 2.3-fold increased expression levels. However, the positive effect of the cHS4 insulator on transgene 5 cHS4 Insulation of Transposon-Delivered SCD-inhibitor Transgenes expression levels was most pronounced in clones with a high copy number, where the increase in mean MFI was observed to be as high as 3.7-fold for SB clones and 2.7-fold for PB clones at day 56 of passage. In Tol2 clones, mean MFI values were consistently lower for clones carrying insulated transposons compared to clones containing uninsulated transposons, suggesting that incorporation of cHS4 sequences into the Tol2 vector did not lead to a protection against repressive position effects. In summary, these results indicate that the cHS4 insulator did not have a long-term stabilizing effect on transgene expression from SB, PB, and Tol2 vectors in ARPE-19 cells, but that inclusion of cHS4 sequences had an overall beneficial effect on the level of transgene expression from integrated SB and PB transposon vectors. 6 cHS4 Insulation of Transposon-Delivered Transgenes Increased transposition of insulated SB transposon vectors in ARPE-19 and HeLa cells In the context of SB vectors, the stable transfection rate of cHS4-insulated vectors is higher than the uninsulated counterpart. The number of resistant colonies obtained in colony formation assays is affected by several factors, including transfection efficiency, transposition activity, and expression levels of the resistance gene. To investigate in further detail, if the increase 22803826 in stable transfection rate observed for the pSBT/ cHS4.RGIP.cHS4 vector was due to better transposition of the insulated element, we performed a quantitative PCR assay in ARPE-19 cells to quantify excision circles formed after transposon mobilization from plasmid DNA. A SB100X- or iPB-expressing plasmid, in which the ampicillin resistance gene had been replaced by a chloramphenicol resistance gene, was transfected into ARPE-19 cells together with insulated or uninsulated transposon plasmid. Transfections of transposon plasmid in the absence of transposase were included as negative controls. Two days after transfection, low-molecular weight DNA was extracted, and real-time qPCR analysis was performed using a primer set flanking the transposon excision site. To account for variations in template DNA input, a PCR specific for the Amp gene in the transposon plasmid backbone was utilized to normalize the excision circle data to the amount of transposon plasmid recovered after transfection. Whereas no difference in the amount of excision circle products was observed for transfections with PBT/RGIP and PBT/cHS4.RGIP.cHS4, a 1.3-fold increase in excision circle formation was observed for transfections with SBT/cHS4.RGIP.cHS4 compared to transfections with SBT/ RGIP. Although this 
increase was not statistically significant, we reproducibly observed higher levels of excision from the SB vector harboring the insulators, suggesting that 10884520 the increased stable transfection rate, obtained by flanking insulator sequences in the SB transposon vector, was partly caused by an increased level of transposon mobilization. To validate this finding in another cellular context, we investigated stable transfection rates and plasmid mobilization of SBT/RGIP
