<p>Disclosed is a preparation of microparticles made up of a polymeric matrix and a nucleic acid expression vector. The polymeric matrix includes one or more synthetic polymers having a solubility in water of less than about 1 mg/l. At least 90 % of the microparticles have a diameter less than about 100 microns. The nucleic acid is either RNA, at least 50 % of which is in the form of closed circles, or circular DNA plasmid molecules, at least 50 % of which are supercoiled.</p>
