IDO1 and esophageal squamous cell carcinoma: 5‐FU, 5‐fluorouracil; CAD, carbamoyl‐phosphate synthetase 2, aspartate transcarbamoylase, and dihydroorotase; CRC, colorectal cancer; DHFR, dihydrofolate reductase; DNA, deoxyribonucleic acid; ESCC, oesophageal squamous‐cell carcinoma; GART, glycinamide ribonucleotide transformylase; GEC, gastroesophageal cancer; IDO, indoleamine 2, 3‐dioxygenase 1; NCT, The National Clinical Trial number; NSCLC, non‐small cell lung cancer; PD‐1, programmed death protein‐1; PD‐L1, programmed death ligand‐1; TS, thymidylate synthase.