Non-proteinogenic amino acids are amino acids that are not incorporated into proteins during the process of protein synthesis. Unlike the 20 standard amino acids that make up proteins, these amino acids can serve various roles in metabolism and cellular functions. They can be involved in the synthesis of neurotransmitters, hormones, and other important biomolecules. Some examples of non-proteinogenic amino acids include ornithine, citrulline, and gamma-aminobutyric acid (GABA). These amino acids can be produced in the body or obtained from dietary sources. Their unique structures and functions contribute to a wide range of biological processes beyond protein formation.