IDO1 and esophageal squamous cell carcinoma: The common 18 immune checkpoints (PD1, PD-L1, PD-L2, BTLA, B7H3, CTLA4, IDO1, LAG3, VSIR, TIM3, CD27, CD28, CD40, ICOS, IL2RB, GITR, OX40, 41BB) were matched to the genes measured in 257 ESCC patients.