PDCD1 and neoplasm: Compared with effector T cells, Treg cells can use Monocarboxylate transporter-1 (MCT1) to efficiently uptake lactic acid in tumor microenvironment with abnormally elevated glycolysis levels, promote the entry of NFAT1 into the nucleus and induce Treg cells to express PD-1, while the expression of PD-1 on CD8+T cell will be inhibited, resulting in the failure of PD-1 immunoblocking therapy.