PRF1 and neoplasm: Tregs inhibit normal immune cells in the tumor microenvironment, allowing tumor cells to exist [73] primarily via four mechanisms: (1) induction of T cell apoptosis via cell-cell interactions; (2) inhibition of immune responses by cytokine secretion; (3) release of perforin and granzymes to kill CTLs, monocytes and DCs directly; and (4) suppression of local immune responses through expression of cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) [74–76].