a composition for in vivo transfection of vertebrate male germ cells comprises a nucleic acid or transgene , and a gene delivery system , and optionally a protective internalizing agent , such as an endosomal lytic agent , a virus or a viral component , which is internalized by cells along with the transgene and which enhances gene transfer through the cytoplasm to the nucleus of the male germ cell . a pharmaceutical preparation and a transfer kit utilize the composition . a method for introducing a polynucleotide into vertebrate male germ cells comprises the administration of the composition to a vertebrate . a method for isolating or selecting transfected cells utilizes a reporter gene , and a method for administering transfected male germ cells utilizes male germ cells which have been transfected in vitro .