Identification of a novel cell surface molecule specific to primitive hematopoietic cells. (A) Expression of Tmtsp in hematopoietic cells. Semiquantitative RT-PCR was carried out using normalized cDNA by GAPDH. Cells analyzed are bone marrow CD34^–c-Kit⁺Sca-1⁺Lineage marker^– HSCs (CD34^–KSL), CD34⁺KSL progenitors, lineage marker^– cells (Lin^–), Gr-1⁺ neutrophils, Mac-1⁺ monocytes/macrophages, TER119⁺ erythroblasts, B220⁺ B cells; spleen Thy-1.2⁺ T cells, NK1.1⁺ natural killer (NK) cells, and B220⁺ B cells; and thymic CD4^–CD8^– T cells (DN), CD4⁺CD8⁺ T cells (DP), CD4⁺CD8^– T cells (CD4SP), and CD4^–CD8⁺ T cells (CD8SP). (B) Deduced amino acid sequence and conserved domains of mouse Tmtsp. Tmtsp protein possesses several domains indicated as follows: signal sequence, dotted line; cysteine residues of Ig-like domain, asterisks; thrombospondin type I repeat (TSP1 domain), underline; transmembrane domain, double line; PKC-phosphorylation site, diamonds; N-glycosylation site, crosses; and tyrosine-phosphorylation site, box. The cDNA sequences are available from the GenBank database under the accession numbers AB039946 and AB044385 (mouse and human, respectively). (C) Schematic illustration of Tmtsp (i) and Unc5h3 proteins (ii).