||CHEMICAL KEYS TO AN UNDERSTANDING OF LIFE PROCESSES 11 times even by their own parents. The fact that two such individuals resemble one another in essentially every detail of anatomy, physiology, and hence in their chemistry, is precisely what we are talking about. With identical twins, blood may be transfused, tissues and organs like the kidney may be transplanted with complete impunity because we know that the twins are identical and no harm can result. Thus, differences between individuals, or between individual cells of different organs, or between cells of different species, depend on the presence of different proteins. Such differences may be quantitative in character as among normal individuals, or they may be qualitative as among different species. Whether we are mice or men depends directly on the nature of the enzymes and other proteins which are present and on their specificity. V In order to have some concept concerning the basis of such specificity, the architecture of proteins must be considered. Proteins are large molecules. What is meant by this ? Let us start from the beginning. All of you know that a water molecule has the formula H20 which signifies that it consists of three atoms, two of hydrogen and one of oxygen. The sucrose or sugar molecule has the formula C12H22OH. This means that it contains forty-five atoms of carbon, hydrogen and oxygen arranged in a particular way. Now to a chemist, sucrose is a complex molecule and it was a difficult task to determine the arrangement of all of these atoms and an even more difficult job to synthesize this substance in the laboratory, one that was accomplished just a few years ago. The complexity of protein molecules is far beyond that of the sucrose molecule. The structure of one of the small proteins has now been determined. The naked or elementary formula of bovine insulin is now known just as precisely as we know the formula of the water molecule or of the sugar molecule. The formula for bovine insulin is C254H377N65075S6. To attempt to visualize the details of how these seven hundred seventy-seven atoms are linked to one another seemed an impossible task only a decade ago. Yet only a few years later this was accomplished in England by Sanger and his co-workers. In fact, the structure of the insulins of several species has been determined and we now know precisely how bovine insulin differs from the insulins derived from the pancreas of the hog, sheep, horse, and whale.