KLRK1 and neoplasm: Subsequent studies confirmed and extended these observations (104, 105): TGF-β inhibits NKG2D-mediated lysis of target cells without altering the expression of perforin or Fas ligand, or without affecting NK cell viability, indicating that down-regulation of NKG2D is a major effect of TGF-β on NK cytolysis of tumor cells (105).