Figure 1. The generation of transgenic animals by gene microinjection. The embryos obtained by superovulation or by in vitro fertilization receive the foreign genes and are developed in foster mothers. Transgenes are detected and transmitted to progeny by normal reproduction. PCR, polymerase chain reaction.

Figure 2. The mechanisms leading to the random integration of a foreign gene into an animal genome. The foreign DNA sequences recognize short and partially homologous regions of the genome. Reparation mechanisms integrate the foreign DNA. Before integration, a homologous recombination mechanism generates polymers of the foreign gene organized in tandem.

Figure 3. The experimental protocol leading to specific gene replacement. A gene construct containing two long regions strictly homologous to the targeted host gene and containing a foreign DNA region transferred to cells. The homologous sequences recombine and the targeted gene is replaced by the foreign gene. The cells in which gene replacement occurred are saved by double selection (not shown here).

Figure 4. The transmission of a mutation by the generation of chimaeric animals. The mutipotent embryonic cells in which gene replacement occurred are transferred into a recipient embryo and participate in its development. The mutation can be transmitted to progeny.

Figure 5. The transmission of a mutation by the cloning technique. The fetal cells in which gene addition or replacement occurred are used to generate living embryos after transfer into enucleated oocytes. The mutation is transmitted to progeny.