Technically, any protein-based drug is a “therapeutic protein,” but the term was first used to describe medicines that are genetically engineered versions of naturally occurring human proteins. A protein is a large molecule comprised of a long chain of amino acids that is folded into a three-dimensional shape. The specific amino acid sequence and 3-D shape determines the biological function of the protein. Therapeutic proteins can be used to replace a protein that is abnormal or deficient in a particular disease. They can also augment the body’s supply of a beneficial protein to help reduce the impact of disease or chemotherapy. Genetically engineered proteins can closely resemble the natural proteins they replace, or they can be enhanced by adding sugars or other molecules that extend the protein’s duration of activity. Therapeutic protein drugs are an important class of medicines serving patients most in need of novel therapies. Recently approved recombinant protein therapeutics have been developed to treat a wide variety of clinical indications, including cancers, autoimmunity/inflammation, exposure to infectious agents, and genetic disorders. The latest advances in protein-engineering technologies have allowed drug developers and manufacturers to fine-tune and exploit desirable functional characteristics of proteins of interest while maintaining (and in some cases enhancing) product safety or efficacy or both.