Until recently, pharmaceuticals used for the treatment of diseases have been based largely on the production of relatively small organic molecules synthesized by microbes or by organic chemistry. These include most antibiotics, analgesics, hormones, and other pharmaceuticals. Increasingly, attention has focused on larger and more complex protein molecules as therapeutic agents. Proteins are large molecules composed of long chains of subunits called amino acids (see Suslow, Thomas, and Bradford

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... 002) . Just as words are composed of the 26 letters of the alphabet, proteins are composed of different combinations of the 20 or so amino acids, except that the length of proteins is often 100 to 1,000 amino acids ("letters") long. The structure and functionality of a given protein is determined by its sequence of amino acids, which, in turn, determines its three-dimensional conformation, or structure. Internal bonds (sulfur and hydrogen bonds) among the amino acids give the protein its final shape and form. Complex proteins undergo further processing such as the addition of phosphate groups (phosphorylation) or carbohydrate molecules (glycosylation), which modify the proteins' functions. Information stored in DNA directs the protein-synthesizing machinery of the cell to produce the specific proteins required for its structure and metabolism. Since proteins play critical roles in cell biology, they have many potential therapeutic uses in preventing and curing diseases and disorders. The first protein used to treat disease was insulin, a small peptide that revolutionized the treatment of diabetes. In addition, the antigens used in vaccinations to induce immune responses are often proteins.