The neurotransmitter serotonin [5-hydroxytryptamine (5-HT)] is causally involved in multiple central nervous facets of mood control and in regulating sleep, anxiety, alcoholism, drug abuse, food intake, and sexual behavior (1). In peripheral tissues, 5-HT regulates vascular tone, gut motility, primary hemostasis, and cellmediated immune responses (1). 5-HT is synthesized in two steps, with tryptophan hydroxylase (TPH) as the rate-limiting enzyme (2). TPH belongs to a superfamily of aromatic amino acid hydroxylases, together with phenylalanine(PAH) and tyrosine hydroxylases (TH), and has been detected mainly in the brain stem and gut enterochromaffin cells (2). To study the physiological impact of the loss of 5-HT synthesis, we generated mice genetically deficient for TPH (Tph/). Like wildtype (Tph/) siblings and the mouse strains from which the Tph/animals were derived, Tph/mice still expressed normal amounts of 5-HT in classical serotonergic brain regions. However, Tph/mice lacked 5HT in the periphery except for in the duodenum (Fig. 1A), which contained about 4% of normal 5-HT levels, probably because of the serotonergic neurons in this tissue (3). Concordantly, Tph/mice exhibited no significant behavioral differences in elevated plus maze and hole board tests (table S1), which are indicative for 5-HT–related behavior. Despite suggestions of a possible second TPH isoform (4), molecular verification has been lacking. Therefore, we screened the High

Throughput Genomic Sequences database of GenBank with short translated sequences of Tph and obtained a human genomic clone on chromosome 12, with an open reading frame similar to Tph exon 4, but different from Pah, which is located on the same chromosome. On the basis of this sequence, we performed 5-and 3-RACE experiments with brain RNA of Tph/mice and obtained the full-length cDNA (referred to as