Two approaches for exploiting alloreactive NK cells. NK cells from a KIR-KIRL mismatched alloreactive donor or from umbilical cord blood (UCB) can be transferred directly or after ex vivo expansion after mild chemotherapy with transient NK cell proliferation. This will induce only a short-lived antileukemic effect. However, a patient can be permanently engrafted with hematopoietic stem cells from an alloreactive donor or UCB after myeloablative therapy, thus establishing a permanent donor KIR phenotype and antileukemic effect. TcRαβ, αβ T-cell receptor.