Identification of inhibitory killer Ig-like receptor (KIR) genes and their cognate human leukocyte antigen (HLA) class I ligands in donor-recipient pairs for patients undergoing haematopoietic stem cell transplantation (HCT) as treatment for haematopoietic malignancies has recently gained considerable interest. One incentive for these studies is to identify potential donors who are not HLA identical with the recipient but who still could serve as acceptable and potentially preferred donors based on their KIR genotype. It is demonstrated that a majority of individuals have at least one inhibitory KIR gene for which they lack the cognate HLA class I ligand. Therefore, the clinical benefits conferred by 'missing KIR ligand' might not be limited only to HLA mismatched donor-recipient combinations but may be applicable also to HLA identical transplants and even autologous HCT.