There are three different methods to prepare transgenic animals:
1. Microinjection of DNA into pronucleus of zygote.
2. Retroviral vectors and retroviral infection of embryo (see Note 1).
3. Embryonic stem cell injection into blastocysts (see Note 2).
Transgenic animals are generally prepared by the micromanipulation of preimplantation embryos. Fig. 1 shows various stages of preimplantation embryo. The most commonly used method is direct microinjection of DNA into one of the pronuclei of a newly fertilized embryo (Fig. 1 A). The overall strategy is shown in Fig. 2. Although this method is the most efficient to introduce foreign genes into the animal genome it has certain limitations as well (see Note 3).
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