When recombinant DNA — DNA deliberately coupled from different sources — was first produced, it was seen as a dangerous example of molecular tinkering gone wild, with potentially disastrous consequences. Two decades later, recombinant DNA, now virtually a household term, has become the most powerful tool of modern genetics. Recent progress in the application of this technique has created radically new possibilities for diagnosis and treatment and has changed the course of medical research. These advances have far-reaching consequences for the future of medicine. Yet the basic principles of recombinant DNA, like the structure of DNA itself, are surprisingly simple. © 1994, Massachusetts Medical Society. All rights reserved.