It is expressed only in select tissues (mucosal tissues, placenta, eye, and pancreas) and by a small population of immune cells (DCs and eosinophils).29,122 However, data confirmed IDO upregulation in CRC, breast cancer, prostate cancer, and esophageal cancer.29 As such, expanding preclinical and clinical data correlated IDO upregulation with suppression of T cell function and ICI resistance.29,122,124. Here, IDO1 is linked to breast cancer.