The present invention provides a polymer modified nucleic acid vector in which the nucleic acid vector is covalently linked to a polymer, which polymer comprises one or more positively charged quaternary amino groups, wherein the nucleic acid vector is a micro-organism selected from the group consisting of a virus, a bacteria or a bacteriophage, a fungus, a spore, a eukaryotic cell nucleus or other micro-organism fragment or a component containing genetic information, and wherein (a) the polymer and/or the linkages between it and the nucleic acid vector are hydrolytically, reducibly or enzymatically degradable； and/or (b) wherein each of the positively charged quaternary amino groups is linked to the polymer backbone via one or more degradable or biodegradable linkages.
