IDO1 and esophageal cancer: suggested that constitutive IDO1 expression in human tumor cells was sustained by an autocrine aryl hydrocarbon receptor (AhR)-IL-6-STAT3 signaling loop (23), although the clinical data revealed that the upregulated expression of IDO1 in various human tumor tissues, such as esophageal cancer, thyroid carcinoma, and leiomyosarcoma, was considered to be a worse prognostic factor and a more aggressive tumor phenotype (24–26).