Vectors used in gene therapy require an expression cassette. The expression cassette consists of three important components: promoter, therapeutic gene and polyadenylation signal. The promoter is essential to control expression of the therapeutic gene. A tissue-specific promoter is a promoter that has activity in only certain cell types. Use of a tissue-specific promoter in the expression cassette can restrict unwanted transgene expression as well as facilitate persistent transgene expression. Therefore, choosing the correct promoter, especially a tissue-specific promoter, is a major step toward achieving successful therapeutic transgene expression. Ideally, the elements of the natural promoter region, necessary for obtaining the required level of the gene expression while retaining tissue-specificity, should be known. Also, it is important to understand whether interactions occur between the promoter region and the rest of the vector genome that could affect promoter activity and specificity. To assess this, it is helpful to select a suitable vector system that will be used in further gene therapy studies. Second, have one or several candidate tissue-specific promoters available for use. Third, ideally have an in vitro cell model suitable to evaluate tissue-specificity. Fourth, have a convenient in vivo animal model to use. Fifth, select a good reporter gene system. Next, using conventional recombinant DNA techniques create different promoter constructs with the selected vector system. Lastly, have a suitable transfection method to test the plasmid constructs in both the in vitro and the in vivo models.