Th e therapeutic DNA is held within a sphere of lipids, termed a lipoplex, or within a sphere of polymers, such as polyethyleneimine, termed a polyplex. Lipoplexes and polyplexes act to protect the DNA, facilitate binding to the target cell membrane and also trigger endocytosis of the complex into the cell, thereby enhancing gene expression. Th ese systems can be modifi ed to include a targeting peptide for a specifi c cell type, such as airway epithelial cells [31] . Th ese complexes effi ciently and safely transfect airway epithelial cells [31] , and they have demonstrated promise in human studies [32] .