PDCD1 and skin squamous cell carcinoma: In cSCC, there is an excessive expression of these molecules, especially programmed cell death protein 1 (PD-1), programmed cell death ligand 1 (PD-L1), cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4), and epidermal growth factor receptor (EGFR), which are molecules that can be therapeutically targeted [95].