Programmed cell death protein 1 (PD-1) expressed on (activated) T-cells and the corresponding programmed death ligand 1 (PD-L1)expressed on immune and tumor cells represent a prominent inhibitory immune checkpoint system that has been demonstrated to play a major role for example in malignant melanoma [2, 3] and squamous non-small cell lung cancer [4, 5]. The gene discussed is PDCD1; the disease is melanoma.