A small protein — a chain of amino acids, shorter than a full protein, that acts as a biological signal or structural molecule.By convention, molecules with fewer than ~50 amino acids are called peptides; larger molecules are proteins — though this boundary is approximate. Peptides can be linear or cyclic, naturally occurring or synthetic, and range from 2 amino acids (dipeptides) to 49 amino acids. Most peptide hormones are 3–40 amino acids. Shorter peptides are more susceptible to proteolytic degradation; modifications like cyclization and D-amino acid substitution are used to improve stability. Not all molecules marketed as "peptides" meet this definition — verify structure when evaluating claims.