PDCD1 and neoplasm: Programmed cell death protein 1 (PD-1)/programmed death-ligand 1 (PD-L1) axis disruption depends on dimethylarginine dimethylaminohydrolase 1 (DDAH1) activity during metastasis in patients with severe gastritis, either synergizing with NO or inhibiting PD-1/PD-L1 activation in tumor growth.