In the four years since the first edition of this book was published, the molecular revolution has continued. DNA has been named by Time maga zine as the Molecule of the Year, a Nobel Prize has been awarded to a young man for the invention of the polymerase chain reaction, and televi sion viewers have learned of the DNA fingerprint. Molecular technology in medicine is increasing. The availability of DNA probes for cancer suscepti bility is stressing our system of insurance, testing our ideas about medical ethics, and teaching us new things about cancer. In this edition, I have added a number of new sections, as well as a new chapter. New examples of molecular medicine have been added to demonstrate current applications of this technology. The basic concepts of molecular biology remain the basis for the first three chapters of the book. The excitement surrounding molec ular medicine that I mentioned in the preface to the first edition continues. It is now tinged with a touch of awe and a little bit of fear at the changes that recombinant DNA technology has brought to our society. v Preface to the First Edition This book describes the discoveries that have created a field called molecu lar medicine. The use of recombinant DNA technology in medical research and most recently in medical practice constitutes a revolutionary tool in our study of disease.