Efficient and prolonged hCFTR expression is one of the major obstacles for cystic fibrosis lung therapy. hCFTR mRNA expression levels depend on eukaryotic expression cassette components, prokaryotic backbone elements, and the gene transfer method may also influence transcriptional silencing mechanisms. A codon-optimized and CpG-reduced human CFTR gene (CO-CFTR) was made. Various vector modifications were tested to facilitate extended duration of CO-CFTR expression. Insertion of an extended 3′BGH transcribed sequence (712 bp) in an inverted orientation produced prolonged expression of CO-CFTR expression at biologically relevant levels. Further studies revealed that prolonged CO-CFTR expression is dependant on the orientation of the extended BGH 3′ BGH transcribed sequence and its transcription, is not specific to the UbC promoter, and is less dependent on other vector backbone elements.