一種生物（通常是老鼠），將外來基因轉入其體內成為其基因組的一部分。引入的基因先被分離出來并設計使其攜帶適當片段。然后將這段基因注入受精卵，方法如下：對一只雌老鼠注射激素使其產生大量卵；讓一只雄老鼠與其交配使部分卵受精；將這些卵收集起來，在其卵裂前注入外來基因物質。這些卵被移植入另一個雌性體內，在那里它們發育成型。在某些卵里基因物質在隨意位點與染色體整合而成為老鼠細胞的遺傳物質。由這種卵發育成的動物將攜帶該基因從而成為轉基因動物。轉基因動物對于描述新發現基因的功能和在大動物體內產生有益蛋白質十分有用。

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).