IDO1 and cancer: Numerous studies have reported that IDO1 is expressed and correlated with prognosis in various cancers, such as urothelial bladder cancer (17), non-small–cell lung cancer (NSCLC ) (19), esophageal squamous cell carcinoma (20), breast cancer (21), pancreatic cancer (22), endometrial cancer (23), and neuroendocrine skin cancer (24).