DNA microinjection methods are used to transfer genes between animals, and are a popular technique for creating transgenic organisms, particularly mammals. A very fine glass pipette is used to manually inject DNA from one organism into the eggs of another. The best time for injection is early after fertilization when the ova have two pronuclei. When the two fuse to form a single nucleus, the injected DNA may or may not be taken up. (The University of California (Irvine) Transgenic Mouse Facility reports an estimated 10-15% success rate based on mice testing positive for transgenes in their experiments). If the DNA is incorporated into the genome, it is done so randomly. Because of this, there is always a chance the gene insert will not be expressed by the GMO, or may even interfere with expression of another gene on the chromosome.
Following DNA injection, the ovum is transferred into the oviduct of a recipient female, or foster mother, that has been induced by mating with a vasectomized male.