Results of first and second HCTs. (A) Quantification of MRD in the bone marrow by flow cytometry. The progressive disease burden seen on relapse after the HCT no. 1 is in contrast to the rapid and sustained disease response following the KIR-mismatched transplantation (HCT no. 2). (B) Flow cytometric evaluation of immune reconstitution after HCT no. 2 showed a rapid NK-cell reconstitution within 3 weeks, whereas T-cell reconstitution remained minimal (the numbers of CD56⁺CD3^- NK cells and CD3⁺CD56^- T cells in each blood sample are shown as the percentage of mean values for 57 healthy children older than 12 months in our institution.) RT-PCR analysis of donor-derived NK cells showed a high level of expression of KIR2DL3; a large subset of KIR2DL3⁺ KIR2DL1^- KIR3DL1^- NK cells was revealed by flow cytometric analysis. Natural cytotoxicity of the donor-derived NK cells against the patients' leukemic blast cells was demonstrated in a standard 2-hour assay⁷  in vitro (mononuclear cell-to-target cell ratio, 40:1). A K562 cell line was used as the standard positive control. RS411 cells, which carried the t(4;11) translocation and expressed all 3 groups of KIR ligands, were used as the negative control.