PDCD1 and melanoma: To date, immune checkpoint (ICP)-targeting drugs, such as anti-cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), anti-programmed cell death protein 1 (PD-1), and anti-PD-1 ligand 1 (PD-L1), have displayed considerable success in a number of cancer immunotherapies,1–4 including melanoma, lung cancer, and other commonly diagnosed cancers.