In the KEYNOTE-590 study, pembrolizumab (a PD-1 inhibitor) combined with chemotherapy significantly prolonged OS compared to a placebo-chemotherapy treatment in the first-line treatment of patients with advanced ESCC (median survival: 12.6 months vs. 9.8 months; hazard ratio (HR) 0.72, 95% Confidence Interval (CI) 0.60–0.88), with tolerable toxicity [9]. Here, PDCD1 is linked to esophageal squamous cell carcinoma.