||20 TWENTY-SECOND ANNUAL REYNOLDS LECTURE laboratories are now studying the simpler nucleic acid systems of viruses to determine the precise change which occurs in a nucleic acid when a mutation occurs and how this is reflected in the strucure of the protein produced under the influence of the specific nucleic acid. Thus we have come full cycle near the end of our present story. The evidence is good that genetic factors are nucleic acids. Somehow, by processes which are only now becoming accessible to investigation, the nucleic acids promote formation of specific proteins. The main functions of proteins in living cells involve their catalytic ability as enzymes and these regulate and determine the functional behavior and metabolism of living cells. Our present picture is that the substances responsible for the behavior of living cells are proteins. The key substances which determine the kind of proteins in cells are nucleic acids. Please do not misunderstand the simplified picture which has been presented. As a result of the activity of proteins and nucleic acids, thousands, if not tens of thousands, of other chemical compounds are involved in the metabolism of an individual cell and for the existence and the reproduction of the cell, all of these other substances are equally vital. Yet the pattern of metabolism and the pattern of reproduction are determined by protein and nucleic acid. The problems involved in studying these large, complex molecules remain very formidable indeed. Many years of work remain before the structure of even a dozen of the smaller proteins will be determined. As yet the tools necessary for attempting the structure of the large protein are not available. The problem of nucleic acid chemistry is in many ways even more difficult. It is evident that in a higher organism there must be a large number of different genetic factors which determine the pattern of inheritance. If each such genetic factor is a nucleic acid we have the immense task of attempting to separate individual, specific nucleic acids from a mixture which contains thousands of similar but closely related molecules. After this has been solved, there remain the problems of attempting to understand the complex, interrelated action of all the nucleic acids and proteins which are involved in development. Just imagine. Each of us began as a single, fertilized egg which contained the complex of genetic factors, the nucleic acids, derived from our parents, and, in addition, a complement of enzymes and other proteins as well as a store of foodstuffs.