Proline is an essential amino acid found in proteins and is an important structural element in the human body. It is an essential amino acid, meaning that it cannot be made by the body and must be obtained from dietary sources. It is a hydrophobic amino acid, meaning that it does not dissolve easily in water. It is also one of the 20 most common amino acids found in proteins and is important for stabilizing the structure of proteins. As a side chain, it is involved in several important biological processes including neurotransmitter release, signal transduction, and apoptosis. Proline has also been associated with regulating gene expression, metabolism, and cell survival.