Although endometriosis is a benign gynecological disease, its biological activities are similar to that of cancer, including metastasis, plantation, angiogenesis, immune tolerance, and recurrence.31, 32 In the present study, we focused on IDO1, MRC2, and Treg cells, which have been reported in previous cancer studies,17, 33–, 35 to identify the endocrine-immune mechanism for the high percentage of peritoneal Treg cells in patients with endometriosis. Here, IDO1 is linked to endometriosis.