The scarcity of effector T cells in the tumor is compounded by enrichment of regulatory T cells (Treg), a subpopulation of CD4+ T cells that suppress effector T cells through cytotoxic T lymphocyte–associated protein 4 (CTLA-4) signaling and secretion of cytokines TGF-β and interleukin-10 [41, 42]. Here, CTLA4 is linked to neoplasm.