<qa>  What type of donor is a human leukocyte antigen?In this study, participants with high-risk hematologic malignancies undergoing hematopoietic cell transplantation HCT, who do not have a suitable human leukocyte antigen HLA-matched relatedsibling donor MSD, matched unrelated donor MURD or killer-immunoglobulin receptors KIR ligand mismatched haploidentical donor identified, will receive an umbilical cord blood transplantation UCBT using a myeloablative preparative regimen. 