The ability to introduce foreign genes into the germ line and the successful expression of the inserted gene in the organism have allowed the genetic manipulation of animals on an unprecedented scale. The information gained from the use of the transgenic technology is relevant to almost any aspect of modern biology including developmental gene regulation, the action of oncogenes, the immune system, and mammalian development. Because specific mutations can be introduced into transgenic mice, it becomes feasible to generate precise animal models for human genetic diseases and to begin a systematic genetic dissection of the mammalian genome.