PDCD1 and esophageal squamous cell carcinoma: Despite recent advances in the personalized therapies, molecular subtyping and development of targeted drugs such as epithermal growth factor receptor (EGFR) inhibitor Gefitinib and anti-programed cell death protein-1 (anti-PD-1) pembrolizumab [3, 4], the prognosis of ESCC remains relatively poor with a 5-year survival rate of below 30% [5].