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Gene therapy is an effective strategy for treating several diseases,and safe and efficient vector systems are the basis for gene therapy applications.The non-viral episomal vectors are preferred as a gene transfer tool because they do not integrate into the host cell chromo-some,which can avoid insertion mutation to prevent host immune response and potential tumorigenicity during disease treatment.In our previous study,we constructed an episomal vector driven by human cytomegalovirus major immediate-early(CMV)promoter and mediated by the characteristic motifs of scaffold/matrix attach-ment regions(S/MARs)[1].The vector contains only 387-bp MARs instead of the 2200-bp S/MARs in the original non-viral episo-mal vector pEPI,which showed the higher transgene efficiency and expression level[2].However,the transgene expression stability and level were still unsatisfactory[1].It has been found that sev-eral cis-acting regulatory elements can improve the expression level and stability of protein of interest,and the effect of the regula-tory elements is affected by other vector elements[3].Chicken hypersensitive site 4(cHS4)is an insulator from the 5'end of chicken β-globin locus,exhibiting excellent insulator properties[4].