The transcribed regions of the genes contain a number of regulatory elements that control exon splicing, mRNA transfer to cytoplasm translation, mRNA targeting, and mRNA half-life. A vector for transgene expression must contain at least one intron, which is required for the transfer of the mRNA to cytoplasm, although several introns may be present. The intron may be added before or after the cDNA. Alternative splicing is a quite frequent event in higher-vertebrate mRNA to control gene expression and to enhance the diversity of the cellular mechanisms using a limited number of genes. The association of introns and cDNAs often leads to alternative splicing, which prevents the transgene from driving the synthesis of the expected protein. A model gene construct using whey acid protein promoter is shown below:
Fig. WAP (whey acidic protein) construct for expression in mammary gland.