In tumor cells, the anti-tumor efficacy of effector T-cells can be retarded by Tregs as the upregulation of co-inhibitory receptors, including cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4), programmed cell death protein 1 (PD-1), T-cell immunoglobulin and mucin-domain containing-3 (TIM-3) and lymphocyte activation gene-3 (LAG-3). Here, PDCD1 is linked to neoplasm.