Another phase I study showed that multiple doses of NK cells (days—2, 7 and 28 post-HCT) expanded ex vivo with K562-mbIL21-41BBL feeder cells, which were genetically modified K562 leukemia cell line expressing membrane-bound IL-21 and the 41BB ligand, could be effective in controlling leukemia relapse as well [82]. Here, IL21 is linked to leukemia.