Oxford dict. of biochemistry and molecular biology, 2006 (protein: any of a large group of organic compounds found as major macromolecular constituents of living organisms. A protein is a linear polymer of amino acids linked by peptide bonds in a specific sequence. In the biosynthesis of the polypeptide chains, any of 20 different amino acids may be incorporated according to the genetic instructions of the cell.) http://id.loc.gov/authorities/subjects/sh85107666

Facts on file dict. of biochemistry, 2003 (protein: one of a large number of substances that are important in the structure and function of all living organisms. Proteins are polypeptides; i.e., they are made up of amino acid molecules joined together by peptide links. Their molecular weight may vary from a few thousand to several million. About 20 amino acids are present in proteins. Proteins may consist of one or several polypeptide chains.)