This gene encodes a member of the pterin-dependent aromatic acid hydroxylase family. The encoded protein catalyzes the first and rate limiting step in the biosynthesis of serotonin, an important hormone and neurotransmitter. The human genome contains two related tryptophan hydroxylases, one on chromosome 11p15-p14 and one on chromosome 12q21. This gene is expressed predominantly in the brain stem. Mutations in this gene may be associated with psychiatric diseases such as bipolar affective disorder and major depression.Bader M., Science 299:76-76(2003).Gibbs R.A., Nature 440:346-351(2006).Pandey A., Proc. Natl. Acad. Sci. U.S.A. 104:2199-2204(2007).