PDCD1 and hematologic disorder: To date, the US Food and Drug Administration (FDA) has approved six immune checkpoint inhibitors blocking cytotoxic T lymphocyte-associated antigen 4 (CTLA-4), programmed cell death protein 1 (PD-1) or programmed cell death ligand 1 (PD-L1), and the second CAR-T cell therapy for the treatment of solid and hematological malignancies.