Serotonin, or 5-hydroxytryptamine (5-HT), is a mono-amine neurotransmitter derived from tryptophan that is responsible for regulation of mood, commonly referred to as the "depression gene". It is primarily located in the CNS and GI tract, and modulates physiological functions related to neurotransmission, GI motility, pain perception, hemostasis, hunger, and cardiovascular integrity.