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Top of pageAbstract The development of immortalized airway epithelial cells has been a significant contribution to the elucidation of the biochemical and molecular mechanisms that underlie cystic fibrosis (CF) pathology and in the development of CF therapies. The different cell systems have been employed in genetic complementation studies as well as the characterization of CF transmembrane conductance regulator (CFTR) related metabolic pathways. An important feature of CF cells complemented with wild-type (WT)-CFTR is reversal of the cAMP-dependent Cl ion transport defect in these cells. Such complementation studies have led to the understanding of numerous relationships between CFTR function and specific biochemical pathways. However, there is very little known about the affect of the degree of CFTR expression on CFTR related functions. In particular, the level of vector-driven WT-CFTR expression has not been well compared to |[Delta]|F508-CFTR expression driven by the same expression vector in airway epithelial cells. Furthermore, many of the complemented CF cells have been complemented with CFTR cDNA that is only comprised of the |[sim]|4.7 kB open reading frame (ORF) rather that the entire |[sim]|6.2 kB CFTR cDNA comprised of not only the ORF, but also the 3' and 5 ' untranslated regions (UTR). The studies presented describe CF airway epithelial cell culture systems that have been complement with the 4.7 kB and 6.2 kB WT-CFTR as well as the 4.7 kB |[Delta]|F508-CFTR under the regulation of a cytomeglovirus (CMV) promoter and cloned into an episomal expression vector. Several different cell lines that are either homozygous for the |[Delta]|F508 mutation or are compound heterozygotes for the |[Delta]|F508 and Q2X alleles have been developed and evaluated for their ion transport characteristics as it relates to their CFTR expression levels. Of particular note, is the relationship between the degree of |[Delta]|F508-CFTR expression and level of the levels cAMP-dependent Cl transport observed. In one set of cells complemented with the |[Delta]|F508-CFTR that is expressed at levels 5-10 fold higher than those observed for WT-CFTR (6.2kB), appeared to result in |[sim]|25% of cAMP-dependent Cl transport observed for the WT-CFTR. The correlation between the expression of the 6.2 kB or the 4.7 kB WT-CFTR and ion transport appear, at present, to be associated with the plasmid number in the cells. Further studies are underway to more fully correlate the degree of CFTR expression with the ion transport characteristics of the complemented cells. These studies will be important in assessing the degree of transgene expression required to achieve and maintain wild-type function in mutant cells.