Nitric oxide (NO) and serotonin are two signaling molecules that correlate to human aggression levels. Nitric oxide is a peptide hormone that is produced when the nitric oxide synthase (NOS) binds oxygen and arginine. The two NOS isoforms that create NO in this case are neuronal NOS (nNOS) and endothelial NOS (eNOS). Serotonin is a steroid hormone that is synthesized in the central nervous system (CNS) by the brain. Serotonin, or 5-hydroxytryptamine (5-HT), is created when the amino acid, tryptophan, combines with the chemical reactor, tryptophan hydroxylase.