Considering the higher peritoneal Treg cell percentage in EMS patients than that in healthy women, as well as findings that IDO1 controls Treg cell function in response to inflammatory stimuli,14, 15 and higher expression of IDO1 in ectopic lesion regulates biological activities of ESCs in endometriosis,6 we reasoned that excessive estrogen may regulate IDO1 expression in the ectopic lesion to induce Treg cell differentiation. The gene discussed is IDO1; the disease is endometriosis.