一种生物（通常是老鼠），将外来基因转入其体内成为其基因组的一部分。引入的基因先被分离出来并设计使其携带适当片段。然后将这段基因注入受精卵，方法如下：对一只雌老鼠注射激素使其产生大量卵；让一只雄老鼠与其交配使部分卵受精；将这些卵收集起来，在其卵裂前注入外来基因物质。这些卵被移植入另一个雌性体内，在那里它们发育成型。在某些卵里基因物质在随意位点与染色体整合而成为老鼠细胞的遗传物质。由这种卵发育成的动物将携带该基因从而成为转基因动物。转基因动物对于描述新发现基因的功能和在大动物体内产生有益蛋白质十分有用。

An organism (typically a mouse) that is engineered to carry a foreign gene, or transgene of choice, as part of its own genetic material. The gene to be introduced is first isolated and engineered (using the techniques of genetic engineering) to carry appropriate elements. The transgene of choice is then injected into a fertilized egg, which is obtained as follows; A female mouse is injected with several hormones to cause it to produce numerous eggs; The female is then mated with a male so that some of the eggs are fertilized; the eggs are collected before they divide and are injected with the foreign genetic material. These eggs are then transplanted back into another (recipient) female, in which they can develop to term. In some of the eggs, the genetic material integrates at a random site on a chromosome and so becomes part of the mouse cell’s genetic material. The animal resulting from that egg will therefore carry that gene and so is a transgenic animal. Trangenic animals are very useful for delineating the function of newly discovered genes as well as for producing useful proteins in large animals (e.g. the production of alpha anti-trypsin in goat’s milk).