KLRK1 and central nervous system cancer: The NKG2D system is suppressed by transforming growth factor (TGF)-β, a master immunosuppressive cytokine expressed by glioma cells, by at least three mechanisms: TGF-β (i) down-regulates the expression of NKG2D on immune effector cells, (ii) reduces MICA and ULBP2 expression levels on the surface of glioma cells [7-9], and promotes NKG2DL protein cleavage from the cell surface in a metalloproteinase-dependent manner [8].