PDCD1 and skin squamous cell carcinoma: These observations are biologically plausible: chronic ultraviolet exposure on sun-exposed skin drives a very high tumour mutational burden and abundant neoantigens, and head-and-neck cSCC often exhibits increased PD-L1 expression and an inflamed tumour microenvironment, which may enhance susceptibility to PD-1 blockade [110,111,112,113,114].