The present disclosure provides methods of generating multiplexed genetically modified animals, for example, porcine endogenous retrovirus (PERV)-inactivaied pigs. The disclosure also provides methods of improving the birth rate of multiplexed genetically modified animals. In some embodiments, the present disclosure is concerned with the generation and utilization of porcine cells in which porcine endogenous retroviral (PERV) elements have been inactivated. In sonic embodiments, the PERV-free or PERV-reduced porcine cells are cloned to produce porcine embryos. In some embodiments, the PERV-free or PERV-reduced embryos may be grown into adult swine from which organs and/or tissues may be extracted and used for such purposes as xenotransplantation into non-porcine animals such as humans.
