Indoleamine 2, 3-dioxygenase 1 (IDO1) is a rate-limiting enzyme that converts the essential amino acid tryptophan into downstream kynurenine (Kyn), indicated as a novel mechanism for blocking the proliferation and anti-tumor activity of immune cells by activating the aryl hydrocarbon receptor (AhR) [4, 5]. The gene discussed is AHR; the disease is neoplasm.