NKG2D activation is an important factor in tumor recognition and lysis by Vγ9Vδ2+ T cells, potentially playing a costimulatory role in cooperation with TCR-dependent activation (37, 41), although direct ligation of the Vγ9Vδ2+ receptor by the NKG2D ligand ULBP-4 has been reported (42). This evidence concerns the gene KLRK1 and neoplasm.