Figure 1 illustrates the mechanism of anti-PD-1 therapy. Clinical trials in patients with melanoma, colorectal cancer, prostate cancer, non-small-cell lung cancer, and renal cell carcinoma established the benefit of the PD-1/PD-L1 blockade in humans and have revolutionized the treatment of patients with these cancers [41]. Here, PDCD1 is linked to melanoma.