PDCD1 and Merkel cell skin cancer: Lymphocytes of the tumor microenvironment exhibit an “exhausted” phenotype with increased expression of programmed cell death protein 1 (PD-1) and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) [31] and an increase in CD4+ and CD8+ regulatory T cells (Treg), confirming that loss of immune-surveillance is a key point in the immunoediting process of MCC [31].