 The success of gene therapy depends on the efficient insertion of therapeutic genes at the appropriate chromosomal target sites within the human genome, without causing cell injury, oncogenic mutations or an immune response.The construction of plasmid vectors is simple and straightforward.Custom-designed ZFNs that combine the non-specific cleavage domain (N) of "Fok"I endonuclease with zinc-finger proteins (ZFPs) offer a general way to deliver a site-specific DSB to the genome, and stimulate local homologous recombination by several orders of magnitude.