Making transgenic animals

There are three main ways to make transgenic animals.

Pronuclear microinjection. Just after fertilisation, the egg holds two pronuclei: a small female one and a larger male one. The transgene is injected into the larger male pronucleus with a fine needle. The embryos are then placed in a foster mother. The method is simple and reliable, but few embryos succeed, and the gene joins the chromosome at a random place.

Embryonic stem (ES) cells. ES cells come from the inner cell mass of the blastocyst, the early embryo. They can become any kind of cell, including the germ cells (eggs and sperms).

• The rDNA is put into ES cells by electroporation.
• The few cells that took it up are selected and injected back into a blastocyst.
• The blastocyst is implanted into a foster mother.

The offspring are chimeras: a patchwork of normal cells and genetically changed ones (for example, cells where a gene has been 'knocked out'). In mice, this often shows as a spotted coat. Chimeric mice are used to study development, gene function, disease and repair.

Retrovirus-mediated transfer. Retroviruses have RNA genomes and naturally insert a DNA copy of themselves into the host's chromosome. Retroviral vectors, such as the lentivirus, can stably insert a single copy of a transgene into early embryos or ES cells.

An example from goats: ear cells are given the human glucocerebrosidase gene; the nucleus of a transgenic cell is put into an egg whose own nucleus was removed; the embryos are placed in an ordinary goat; and the human protein is collected from the milk of the transgenic offspring.