PDCD1 and cancer: In particular, immune checkpoint inhibitors (ICIs) such as anti-programmed cell death 1 (PD-1) antibody (Ab) and anti-cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) Ab have been reported for their revolutionary clinical outcomes [[2], [3], [4]]; they have become standard therapeutic agents for several types of cancer [5].