PDCD1 and metastatic melanoma: Immune checkpoint blockade increases survival in patients with metastatic malignant melanoma, renal cell carcinoma, and lung cancer.1–4 Double immune checkpoint blockade activates T cells by blocking the inhibitory receptors programmed cell death 1 (PD-1) and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) simultaneously, thereby promoting T cells to kill cancer cells.