PDCD1 and esophageal squamous cell carcinoma: Compared with conventional chemotherapy, second-line PD-1 inhibitors significantly improved the OS (hazard ratio [HR] = 0.73, 95% confidence interval [CI]: 0.66–0.81; P < 0.001) and ORR (relative risk [RR] = 1.89, 95% CI: 1.16–3.05; P = 0.01) of advanced ESCC patients, especially significantly prolonged the OS in the patients with positive programmed death-ligand 1 (PD-L1) status (HR = 0.64, 95% CI: 0.53–0.77; P < 0.001); but did not better PFS (HR = 0.88, 95% CI: 0.68–1.14; P = 0.330) and DCR (RR = 0.89, 95% CI: 0.59–1.37; P = 0.603).