PDCD1 and cancer: Furthermore, recent studies have mainly focused on the adaptive immune response and modulation of cancer due to evolutionary discoveries on immune checkpoints, such as programmed cell death protein 1 (PD-1) and cytotoxic T-lymphocyte antigen 4 (CTLA-4), and their application in the clinic; however, the relationship between the innate immune response and cancer treatment has been less investigated 17.