PDCD1 and neoplasm: Given the crucial role of immune checkpoints in suppressing the anti-tumor immune response, the use of monoclonal antibodies (mAbs) as immune checkpoint inhibitors targeting CTLA-4 and PD-1/PD-L1 appears very promising, with their mechanism of action shown in Figure 1. Examples of approved immune checkpoint inhibitors include ipilimumab, an anti-CTLA-4 mAb, and nivolumab, an anti-PD-1 mAb.