PD-1 is highly expressed in activated lymphocytes, whereas PD-L1 is expressed in cancer and stromal cells; binding of PD-L1 to PD-1 suppresses antitumor immune responses.[39] Thus far, the FDA has approved the PD-1/PD-L1 inhibitors nivolumab and pembrolizumab for melanoma, cemiplimab for cSCC, and pembrolizumab and avelumab for Merkel cell carcinoma. Here, PDCD1 is linked to skin squamous cell carcinoma.