PDCD1 and metastatic melanoma: Immune-based treatment of metastatic melanoma using interleukin (IL) 2-conditioned autologous lymphocytic cells from patients’ blood has existed since the 1980s.8–11 Monoclonal antibody-based blockade of the immune checkpoint inhibitors programmed cell death protein 1 (PD-1) and cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) has further revolutionised the field.