Tryptophan 5-hydroxylase 2 (TPH2) is an isozyme member of the biopterin-dependent aromatic amino acid hydroxylase family. TPH2 is primarily expressed in the serotonergic neurons of the brain, with the highest expression found in the raphe nucleus of the midbrain. TPH2 catalyzes the first and rate limiting step in the biosynthesis of serotonin, an important hormone and neurotransmitter. Genetic variation in TPH2 may influence susceptibility to major depressive disorder (MDD). Defects in TPH2 are the cause of psychiatric diseases such as bipolar affective disorder and major depression.