Natural Killer (NK) cells have raised great interest as potential mediators of selective graft-versus-leukemia effects after allogeneic hematopoietic cell transplantation (alloHCT) since the first descriptions of a reduced risk of relapse and improved survival of patients transplanted from haploidentical related donors missing the human leukocyte antigen (HLA) ligands for inhibitory Killer-cell Immunoglobulin-like Receptors (KIR) (1). The gene discussed is KIR3DL1; the disease is leukemia.