KLRK1 and neoplasm: Similar to K562–mb15–41BBL, K562 genetically modified to express mbIL-21, or to co-express the ligand for 41BB and the NKG2D ligand MICA (K562–4-1BBL–mMICA), have been shown to promote large-scale expansion of NK cells with enhanced anti-tumor in vitro reactivity (121–123).