IDO1 and melanoma: It has been reported that IDO1 is highly expressed in a variety of tumors, including colorectal cancer, breast cancer, esophageal carcinoma, cervical squamous cell carcinoma, melanoma, pancreatic cancer [32, 33], etc., and IDO1 is also expressed in many other cells, such as vascular cells, antigen-presenting cells (APCs, such as macrophages and DCs), eosinophils, endothelial cells (ECs), fibroblasts, and so on [34].