Cryptic protein fragments as an emerging source of peptide drugs.

This commentary evaluates the newly proposed concept of crypteins - peptides with embedded cryptic activity - as a source of peptide drugs. Although several crypteins are undergoing advanced clinical trials, and one cryptein, adrenocorticotropic hormone (ACTH), is already widely used as a therapeutic, crypteins remain an underappreciated class of biopharmaceuticals. The systematic exploitation of crypteins provides a novel source of peptide drugs, and may also provide templates for developing peptidomimetic compounds. In addition, crypteins appear to offer several advantages over synthetic protein libraries and other protein scaffolds.