Abstract: A composition for in vitro and 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 method of genetically altering a vertebrate male germ cell in vivo employs a lentiviral-derived vector. A method of substantially depopulating a vertebrate testis employs a combination of a dose of an alkylating agent, such as busulfan, chlorambucil, cyclophosphamide, melphalan, or ethyl ethanesulfonic acid, and a dose of gamma radiation. 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.