Indoleamine-pyrrole 2,3-dioxygenase (IDO or INDO) is a heme-containing enzyme physiologically expressed in a number of tissues and cells, such as the small intestine, lungs, female genital tract or placenta. IDO is an important part of the immune system and plays a part in natural defense against various pathogens.It is produced by the cells in response to inflammation and has an immunosuppressive function because of its ability to limit T-cell function and engage mechanisms of immune tolerance.Emerging evidence suggests that IDO becomes activated during tumor development, helping malignant cells escape eradication by the immune system. Expression of IDO has been described in a number of types of cancer, such as acute myeloid leukemia, ovarian cancer or colorectal cancer.