PDCD1 and cancer: Furthermore, mAbs targeting co-inhibitory ICs (i.e., PD-1 and CTLA-4) have shown clinical efficacy in several cancers, such as melanoma [23], non-small cell lung cancer (NSCLC) [24], renal and bladder cancers [14], head and neck cancer [25], colorectal carcinoma (CRC) [26,27] hepatocellular carcinoma (HCC) [28], Merkel cell carcinoma [29], and Hodgkin lymphoma [30].