The use of gene targeting to develop animal models for human genetic diseases.

In the early 1980s i t became possible to produce animals carrying experimentally introduced foreign genes in their genome. T h e foreign gene is referred t o as a transgene and the animals as being transgenic. This technology, applied initially to mice and subsequently to other mammals, has been o f considerable benefit to basic biological and medical research (for a review. see [ 11). and promises to have very important practical and commercial applications. Transgenic mice are routinely used in the study o f mammalian gene expression, in particular to identify DNA sequences which control the tissue specific variations in the expression of a given gene. Our knowledge of the actions and interactions o f oncogenes genes that are instrumental in the development of tumours is largely derived from cxperiments involving transgenic mice. T h e usual method for generating transgenic animals involves the microinjection of the chosen gene into the nucleus of a fertilized egg. However. the use o f transgenic animals is complicated and limited by two properties associated with the conventional methods for introducing DNA into the genome. First, the introduced gene can potentially integrate at any site in the genomc. Consequently, undesirable mutations may arise by insertion into an essential gene. Alternatively. a transgene may integrate into a region of the genome which modifies or even abolishes its expression. Secondly. the number o f copies o f the transgene integrated can vary several hundredfold. T h e level o f expression o f an introduced transgene may also be affected by the number o f copies present. Thus, useful as the approach is, the interprctation o f results is often subject to complications. If more precise changes could be made t o the mammalian germ line. it would be possible to modify and study the expression of a gene in its natural chromosomal position in a whole animal. Many human genetic diseases and some c;incers result from mutation of a single gene. By inactivating such genes in mice it might prove possible t o produce mouse models for human diseases and permit evaluation of novel forms o f treatment, such as somatic gene replacement therapy, where functional genes would be introduced in an attempt t o complement the genetic deficiency in an individual [ 2 I.