These nucleic acid polymers act as the genetic code and translators for proteins, which in turn are the molecular building blocks of body tissues, and determine which amino acids go together to form body proteins. DNA is found mainly in the nuclei of cells and carries the genetic message; small amounts of DNA are also found in the mitochondria. RNA helps transfer this genetic message to guide the manufacture of proteins that use all the amino acids either created by the body or extracted from foods. There are supplements containing good levels of nucleic acids, most commonly yeast or organ meats such as calf thymus, which have been recommended to retard aging, improve memory, or improve the immune or other protein functions. However, there is no proof that RNA or DNA, when taken orally, performs any of these fabulous feats. Most of the oral nucleic acid supplement is broken down into purines and pyrimidines, the basic components of RNA and DNA. These purines, such as adenine and guanine, and pyrimidines, such as cytosine, uracil, and thymine, may have some cellular regeneration functions and thus could help slow aging, improve immune functions, and so on. As these components are absorbed, they may aid the production of the body's RNA or DNA, though this has not been proved. Injectable nucleic acids may offer some benefit. These have been used to slow skin aging particularly.