Following the FDA’s approval of pembrolizumab, nivolumab, and cemiplimab as PD-1 inhibitors due to the successful antitumor T-cell responses to PD-1 inhibition in several cancer models [177], pembrolizumab, a humanized IgG4 mAb, was initially approved for metastatic melanoma and non-small-cell lung cancer. Here, PDCD1 is linked to metastatic melanoma.