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We recently cloned mouse Thy28 cDNA (mThy28), which is highly conserved among vertebrates and plants. The mThy28 mRNA is highly expressed in testis, liver, kidney, brain, with moderate expression in thymus, spleen, and heart. In the present study, characteristics of mouse Thy28 protein expression were examined using rabbit anti-mThy28 polyclonal antibody (Ab). Levels of mThy28 protein expression were highest in testis, with moderate expression in liver, spleen, and thymus. The Thy28 protein was mainly located in the nucleus, which was revealed by immunofluorescence microscopy and Western blotting using anti-mThy28 Ab, and transient expression of the mThy28/EGFP fusion gene. Engagement of membrane immunoglobulin with anti-IgM induced down-regulation of human Thy28 expression at both mRNA and protein levels, accompanied by induction of apoptosis in Ramos B lymphoma cells. Expression of protein and mRNA and induction of apoptosis were evaluated by flow cytometry/Western blotting, reverse transcription-polymerase chain reaction, and propidium iodide staining, respectively. Anti-IgM also down-regulated the promoter activity of the mThy28 gene, as demonstrated by luciferase assay. Thus, the anti-IgM-induced down-regulation of the nuclear Thy28 expression appears to correlate with the induction of apoptosis in Ramos B lymphoma cells.