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Minor histocompatibility antigen (mHag) induces and mounts graft-versus-host disease after allogeneic hematopoietic stem cell transplantation. Among several mHags, HA-1 is one that acts alone and is the most studied. It is suggested that HA-1 may be one of the immunodominant antigens inducing not only graft-versus-host disease but also graft-versus-malignancy effects. There are some reports that mHag HA-1–specific cytotoxic T lymphocytes generated from an HA-1–negative donor can lyse HA-1–positive leukemic cells. However, the tissue distribution of HA-1 has been described as restricted to the cells of the hematopoietic lineage.We examined the HA-1 expression in peripheral blood mononuclear cells (PBMNC), leukemia/lymphoma cell lines, solid tumor cell lines, and paired samples of tumor and normal tissues from individual cancer patients by quantitative reverse-transcription polymerase chain reaction.We found that mRNA of HA-1 is expressed in all leukemia/lymphoma cell lines and PBMNC. Most of the leukemia/lymphoma cell lines have the same levels of HA-1 expression as a leukemia/lymphoma cell line, Raji. The expression levels of human PBMNC were 14- to 19-fold higher than those of Raji. Among 32 solid tumor cell lines, 7 showed >50% expression levels compared with Raji.HA-1 expression in the mRNA level is higher in cells of hematopoietic origin, but this tissue distribution is not strictly restricted. Some solid tumor cells and tissues express HA-1 gene equal to hematopoietic cells.