FIGURE 3.

Schematic representation of practical and possible pathways of genetic modification in large animals. To achieve the generation of live founders with desired genetic modifications (pronuclear injection and nuclear transfer are the two primary procedures), the first step is to conduct gene manipulations in a variety of cells or organs, including somatic cells, embryonic cells, embryos, spermatozoa, SSCs and other targeted organs by the use of many tools, such as viral vectors, recombinases, transposons, RNA interference (RNAi), and endonucleases. Through a series of embryonic operations, such as nuclear transfer, genetically modified cells or embryos can produce genetically modified offspring.