Amino acids have many functions in the body. They are the building blocks for all body proteins ?structural proteins that build muscle, connective tissue including skin, bone and other structures, and functional proteins in the form of thousands of metabolically active enzymes. Amino acids provide the body with the nitrogen that is essential for growth and maintenance of all tissues and structures. Aside from these general functions, individual amino acids also have specific functions in many aspects of human physiology and biochemistry. Amino acids serve as precursors for many nitrogenous substances. These include heme, purines, pyrimidines, hormones, and neuro-transmitters, including biologically active peptides. In addition, many proteins contain amino acids that have been modified for a specific function, e.g., calcium binding or collagen cross-linking. L-Cystine is a non-essential amino acid that is formed when two L-cysteine molecules are joined together by a disulfide bridge, formed when L-cysteine is oxidized. L-cysteine can be converted by the body into L-cystine and vice versa as the body requires. L-cystine is a vital component of hair keratin, digestive enzymes, and lactoglobulin. L-Cystine may be a useful dietary adjunct for individuals wishing to supplement their diet with this amino acid.