Pembrolizumab is a humanized monoclonal IgG4 isotype antibody initially approved as monotherapy for the treatment of metastatic melanoma and refractory NSCLC.2 PD-1 is a molecule expressed on the surface of antigen stimulated T-cells, but also in macrophages, B cells, natural killer T cells, and dendritic cells.3 Pembrolizumab targets the PD-1 and blocks interaction between PD-1 and its ligands, PD-ligand 1 (PD-L1) and PD-ligand 2 (PD-L2).2 Under normal conditions, PD-1 and related PD-L1 are expressed on the surface of activated T cells. The gene discussed is PDCD1; the disease is metastatic melanoma.