PDCD1 and skin cancer: Melanoma is a highly aggressive skin cancer, and several immune-related therapies have been approved by the US FDA for its treatment, including interleukin 2 (IL-2), interferon-α (IFN-α), cytotoxic T cell-stimulating cytokine (CTLA-4) and programmed cell death protein 1 (PD-1) blocking antibodies [30], all of which target T cell activation.