Precursor of the histocomplatibility antigen HB-1. More generally, minor histocomplatibility antigens (mHags) refer to immunogenic peptide which, when complexed with MHC, can generate an immune response after recognition by specific T-cells. The peptides are derived from polymorphic intracellular proteins, which are cleaved by normal pathways of antigen processing. The binding of these peptides to MHC class I or class II molecules and its expression on the cell surface can stimulate T-cell responses and thereby trigger graft rejection or graft-versus-host disease (GVHD) after hematopoietic stem cell transplantation from HLA-identical sibling donor. GVHD is a frequent complication after bone marrow transplantation (BMT), due to mismatch of minor histocomplatibility antigen in HLA-matched sibling marrow transplants. HB-1 is presented on the cell surface by MHC class I HLA-B44. This complex specifically elicits donor-cytotoxic T lymphocyte (CTL) reactivity in B-cell acute lymphoblastic leukemia (B-ALL) after treatment by HLA-identical allogenic bone marrow transplantation (BMT). It induces cell recognition and lysis by CTL. However, HB-1 restricted expression in B-ALL cells and not in normal tissues may allow a specific CTL reactivity against B-ALL without the risk of evoking graft-versus-host disease.