In an attempt to identify novel transmembrane molecules expressed on hematopoietic cells, we identified a novel transmembrane protein gene, M83. Cloning of the full-length cDNAs of human and mouse M83 revealed that M83 encodes a type I transmembrane protein with a region containing five hydrophobic segments within the C-terminal part of the protein, suggesting that M83 is a five-span transmembrane molecule, The M83 protein was expressed on the cell surface as a glycosylated protein with a molecular mass of 84 kDa, The M83 gene was localized to human chromosome 16p13.3, mouse chromosome 17B1, and rat chromosome 10q12.3 distal, In human, M83 mRNA was highly expressed in placenta, pancreas, and lymphohematopoietic tissues including peripheral blood, spleen, and bone marrow, Among hematopoietic cells, it was highly expressed in resting T lymphocytes and was downregulated by cell activation, suggestive of its biological role related to the T cell resting status,