Scientists seeking to understand the role of diet in health maintenance began to use rats in their research on nutrient needs. When these animals were fed diets consisting of purified proteins, fats, and carbohydrates, they died. It was found that specific minerals and additional factors, termed accessory food factors by R. G. Hopkins, were present in an unrefined diet and were necessary to sustain life. Among the many functions that vitamins and minerals serve in the body, one stands out in its primacy. That is, the service in gene expression. The purines and pyrimidines are the bases that comprise DNA and RNA. They are synthesized de novo and this synthesis requires, both directly and indirectly, a number of vitamins and minerals. The purines are adenine and guanine while the pyrimidines are cytosine, uracil, and thymine. Uracil is used for RNA synthesis whereas thymine is used mainly for DNA synthesis.