Seldom has a scientific or biomedical break-through evoked the awe, controversy, or sheer incredulity that has accompanied the developments in the field of recombinant DNA technology or more popularly, gene cloning and genetic engineering. Now little more than one generation after Avery, et al1 demonstrated that genes were encoded in DNA and Watson and Crick2 interpreted the structure of these molecules, genes are being cut, manipulated, and recombined to produce unprecedented new insights into genetics and molecular biology and the prospect of gene therapy. These developments have not occurred without anxiety to both scientists and laymen. At the moment, neither the most apocalyptic fears nor the most optimistic dreams appear to be imminent, although I believe that the dreams are closer to fulfillment than the fears. Recombinant DNA technology is already having great impact in hematology, oncology, endocrinology, immunology, and infectious disease and will soon play an important role in other medical subspecialities as well. In none, however, will it have quite the same impact as in genetics because DNA is the material that genetics "is all about." The cloning and study of phenylalanine hydroxylase is one of the first instances in which this technology has important implications in the diseases traditionally classified as inborn errors of metabolism. In order to understand and appreciate the presentation by Woo on phenylalanine hydroxylase as well as the many future papers that will play so vital a role in all of our professional lives, it is necessary to acquire the basic vocabulary of the field.