Tryptophan (abbreviated as Trp or W) is one of the 20 standard amino acids essential in the human diet. It is encoded in genetic code as the codon UGG. The distinguishing structural characteristic of tryptophan is that it contains an indole functional group. Tryptophan is a biochemical precursor for the compounds such as serotonin (neurotransmitter), melatonin (neurohormone) and niacin (essential for cell survival and DNA repair).

 Neurodegenerative Diseases: Tryptophan Metabolism