Wn that S/MAR vectors can replicate episomally irrespective of the promoter used. We confirm and extend this observation using the pUbC-S/MAR vector in Huh7 and MIA-PaCa2 cell lines. We have obtained similar results by using the pEPI-Luc vector – an S/MAR plasmid where 113-79-1 
luciferase expression is driven by the human CMV promoter (data not shown). However, 25033180 a previous study to mark tumour cells genetically with a luciferase transgene driven by the CMV promoter [4] has shown the limitations of this promoter for long-term transgene expression since the CMV promoter is readily inactivated by several host mechanisms such as CpG methylation [11,14,15,16,17]. This limitation has 
been overcome by our study, which demonstrates a sustained expression from the mammalian UbC promoter in combination with an S/MAR element. Differential establishment of cells can account for differences in luciferase expression seen between CP21 supplier animals in each group following administration. Histopathology analysis of the tumours showed the typical tissue morphology expected of PaCa and HCC (Figure 3) and the immunohistochemical analysis showed all tumour cells derived from those injected into the mouse to be luciferase positive (Figure 3). Given this and the long-term transgene expression achieved for 35 days post-injection where a steep increase of expression is observed after 21 days (Figure 2C), this S/MAR vector seems to be ideally suited for use in cancer cell lines to generate a genetically marked murine model of this disease. The maintenance of transgene expression for 35 days is significant and given past in vivo investigations with a similar vector [11], we assume that expression should persist for several more months. Due to associated animal welfare issues, extending the time periodfor this study of tumour models is not feasible and therefore the time period of the study presented here is likely to be fairly representative of most animal tumour model studies. In addition to maintaining long-term reporter gene expression, pUbC-S/MAR was shown to be episomally retained and capable of replication in vitro and in vivo after multiple rounds of cell division confirming previous findings [18,19,23,25,27,28]. Furthermore this paper shows for the first time the ability of an S/MAR vector to replicate episomally in injected tumour cells in vivo. In conclusion, the work presented here highlights the suitability of pUbC-S/MAR pDNA vector as a genetic marker of murine tumour models. In addition to being non-viral in design it is able to facilitate episomal maintenance and long-term transgene expression. Furthermore, our model illustrates the ease and speed in which a vector can be used to stably transfect tumor cells for generating genetically marked tumor models for the development and monitoring of potential therapies in approximately one month. This work can have important applications in the field of anti-cancer drug development for treating HCC or PaCa but also for other cancers, provided that stable cell lines can be generated as shown in the current work.Materials and Methods Ethics StatementAnimal studies were carried out in accordance with UK Research Councils’ and Medical Research Charities’ guidelines on Responsibility in the Use of Animals in Bioscience Research, under a UK Home Office license (PPL# 70/6906; Title: Development of gene transfer vectors as therapeutics and biosensors).Plasmid VectorsThe pUbC-S/MAR (kindly provided by Dr Carsten Rudolph, University of.Wn that S/MAR vectors can replicate episomally irrespective of the promoter used. We confirm and extend this observation using the pUbC-S/MAR vector in Huh7 and MIA-PaCa2 cell lines. We have obtained similar results by using the pEPI-Luc vector – an S/MAR plasmid where luciferase expression is driven by the human CMV promoter (data not shown). However, 25033180 a previous study to mark tumour cells genetically with a luciferase transgene driven by the CMV promoter [4] has shown the limitations of this promoter for long-term transgene expression since the CMV promoter is readily inactivated by several host mechanisms such as CpG methylation [11,14,15,16,17]. This limitation has been overcome by our study, which demonstrates a sustained expression from the mammalian UbC promoter in combination with an S/MAR element. Differential establishment of cells can account for differences in luciferase expression seen between animals in each group following administration. Histopathology analysis of the tumours showed the typical tissue morphology expected of PaCa and HCC (Figure 3) and the immunohistochemical analysis showed all tumour cells derived from those injected into the mouse to be luciferase positive (Figure 3). Given this and the long-term transgene expression achieved for 35 days post-injection where a steep increase of expression is observed after 21 days (Figure 2C), this S/MAR vector seems to be ideally suited for use in cancer cell lines to generate a genetically marked murine model of this disease. The maintenance of transgene expression for 35 days is significant and given past in vivo investigations with a similar vector [11], we assume that expression should persist for several more months. Due to associated animal welfare issues, extending the time periodfor this study of tumour models is not feasible and therefore the time period of the study presented here is likely to be fairly representative of most animal tumour model studies. In addition to maintaining long-term reporter gene expression, pUbC-S/MAR was shown to be episomally retained and capable of replication in vitro and in vivo after multiple rounds of cell division confirming previous findings [18,19,23,25,27,28]. Furthermore this paper shows for the first time the ability of an S/MAR vector to replicate episomally in injected tumour cells in vivo. In conclusion, the work presented here highlights the suitability of pUbC-S/MAR pDNA vector as a genetic marker of murine tumour models. In addition to being non-viral in design it is able to facilitate episomal maintenance and long-term transgene expression. Furthermore, our model illustrates the ease and speed in which a vector can be used to stably transfect tumor cells for generating genetically marked tumor models for the development and monitoring of potential therapies in approximately one month. This work can have important applications in the field of anti-cancer drug development for treating HCC or PaCa but also for other cancers, provided that stable cell lines can be generated as shown in the current work.Materials and Methods Ethics StatementAnimal studies were carried out in accordance with UK Research Councils’ and Medical Research Charities’ guidelines on Responsibility in the Use of Animals in Bioscience Research, under a UK Home Office license (PPL# 70/6906; Title: Development of gene transfer vectors as therapeutics and biosensors).Plasmid VectorsThe pUbC-S/MAR (kindly provided by Dr Carsten Rudolph, University of.