Numerous studies about the potential role of NK alloreactive during a n hematopoietic stem cells graft are based on genotypical analyses of the KIR receptors and on genotypic incompatibilities between KIR and HLA for couple donor/recipient. There is still a lot of issues non resolved: Are KIR really expressed and how occur their expression during time when hematopoietic reconstitution? Is it depending on HLA of the recipient?If KIR are expressed, what are the mechanisms of alloreactivity of NK cells? Are NK able to lyse tumoral cells? Could alloreactive NK cells constitute a therapeutic tool able to induce tolerance and elimination of leukemia during hematopoietic stem cells grafts?