Catalyzes the first and rate limiting step of the catabolism of the essential amino acid tryptophan along the kynurenine pathway . Involved in immune regulation. May not play a significant role in tryptophan-related tumoral resistance .
Detected in liver, small intestine, spleen, placenta, thymus, lung, brain, kidney, and colon . Also expressed at low level in testis and thyroid. Not expressed in the majority of human tumor samples (>99%) .