Food that is taken into the body is ultimately used as fuel, or in the biosynthesis of necessary molecules. These transformations are accomplished by complex, interconnected series of chemical reactions. Diagrammed on paper, these metabolic pathways, are reminiscent of an intricate map of a huge city. Each chemical reaction is controlled by a specific enzyme, of which there are thousands. Because the enzymes are protein molecules, the recipe for each enzyme is encoded by a different gene or part of a gene. If a gene is mutated, the enzyme it encodes will be changed. It may have a low level of function; it may not function at all. A biochemical pathway consists of a series of enzyme-controlled reactions in which the product of one reaction becomes the substrate for the next reaction. There may be many steps in such a pathway. If...