closely related to the machinery for building fatty acids and polyketides, hybrid compounds are often found. The presence of oxazoles or thiazoles often indicates that the compound was synthesized in this fashion. are derived from animal milk or meat digested by proteolysis. In addition to containing small peptides, the resulting material includes fats, metals, salts, vitamins, and many other biological compounds. Peptones are used in nutrient media for growing bacteria and fungi. Peptide fragments refer to fragments of proteins that are used to identify or quantify the source protein. Often these are the products of enzymatic degradation performed in the laboratory on a controlled sample, but can also be forensic or paleontological samples that have been degraded by natural effects. Chemical synthesis Example families The peptide families in this section are ribosomal peptides, usually with hormonal activity. All of these peptides are synthesized by cells as longer "propeptides" or "proproteins" and truncated prior to exiting the cell. They are released into the bloodstream where they perform their signaling functions. Antimicrobial peptides Magainin family Cecropin family Cathelicidin family Defensin family Tachykinin peptides Substance P Kassinin Neurokinin A Eledoisin Neurokinin B Vasoactive intestinal peptides VIP (Vasoactive Intestinal Peptide; PHM27) PACAP Pituitary Adenylate Cyclase Activating Peptide Peptide PHI 27 (Peptide Histidine Isoleucine 27) GHRH 1-24 (Growth Hormone Releasing Hormone 1-24) Glucagon Secretin Pancreatic polypeptide-related peptides NPY (NeuroPeptide Y) PYY (Peptide YY) APP (Avian Pancreatic Polypeptide) PPY Pancreatic PolYpeptideOpioid peptides Proopiomelanocortin (POMC) peptides Enkephalin pentapeptides Prodynorphin peptides Calcitonin peptides Calcitonin Amylin AGG01 Self-assembling peptides Aromatic short peptides Biomimetic peptides Peptide amphiphiles Peptide dendrimers Other peptides B-type Natriuretic Peptide (BNP) - produced in the myocardium and useful in medical diagnosis Lactotripeptides - Lactotripeptides might reduce blood pressure, although the evidence is mixed. Peptidic components from traditional Chinese medicine Colla Corii Asini in hematopoiesis. Terminology Length Several terms related to peptides have no strict length definitions, and there is often overlap in their usage: A polypeptide is a single linear chain of many amino acids (any length), held together by amide bonds. A protein consists of one or more polypeptides (more than about 50 amino acids long). An oligopeptide consists of only a few amino acids (between two and twenty). Number of amino acids Peptides and proteins are often described by the number of amino acids in their chain, e.g. a protein with 158 amino acids may be described as a "158 amino-acid-long protein". Peptides of