CD274 and head and neck squamous cell carcinoma: We undertook the current study to broadly examine mechanisms underlying constitutive and cytokine-induced PD-L1 expression in four human tumor types – melanoma (MEL), renal cell carcinoma (RCC), squamous cell carcinoma of the head and neck (SCCHN), and NSCLC – and to investigate the potential roles of STAT1, STAT3, and p65 activation in driving constitutive and inducible PD-L1 expression on tumor cells and Monos.