PDCD1 and metastatic melanoma: Since 2011, immune checkpoint inhibitors (ICIs), which promote the anti-tumor immune response of activated T cells by blocking the inhibitory signal, including cytotoxic T-lymphocyte associated protein 4 inhibitors, programmed cell death protein 1 (PD-1) inhibitors, and programmed cell death ligand 1 inhibitors, have been used as a treatment method for unresectable or metastatic melanoma, lung cancer, breast cancer, and gastrointestinal tumors with monotherapy or combined therapy (2).