a nucleic acid - containing complex , containing a nucleic acid and a biodegradable polymer , especially a positively - charged water - insoluble biodegradable polymer , is disclosed . the complex has excellent properties of sustainedly releasing a desired nucleic acid , especially dna , to a site in need of a treatment . since the complex can be taken up to phagocytes such as macrophages and delivered specifically to the target site , the function of the nucleic acid can be exhibited in a target site specific manner , and thus more specific gene therapy can be achieved . the complex has no adverse effects , such as occurrence of recombinants or toxicity which could be caused by using a virus vector such as adenovirus , or liposome . thus , the complex is particularly preferable for the field of gene therapy . furthermore , the complex enhances the biological effect of the nucleic acid introduced into the cells , allowing a gene therapy with a lower dose of nucleic acids .