Tryptophan 2,3-dioxygenase (TDO) is an enzyme involved in tryptophan metabolism, responsible for converting tryptophan to N-formylkynurenine, thus initiating the tryptophan degradation pathway. TDO is primarily expressed in the liver and is also found in some other tissues such as the brain and kidneys. TDO plays a crucial role in regulating tryptophan levels, maintaining neurotransmitter balance, and immune regulation. It is involved in various biological responses and signaling pathways, including circadian rhythm regulation, neuroprotection, and anti-inflammatory responses. The specific expression and mutations of the gene are associated with various diseases, including neurodegenerative diseases, tumors, and psychiatric disorders.