The CD28-related inhibitory receptors crucial for T cell regulation, namely cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) and programmed cell-death protein-1 (PD-1), are highly expressed in human SCC samples and associated with cancer progression (Welsh et al., 2009; Gambichler et al., 2017). This evidence concerns the gene PDCD1 and cancer.