MEP21 is an avian antigen specifically expressed on the surface of Myb-EtsCtransformed multipotent hematopoietic precursors (MEPs) and of normal thrombocytes. form a family of stem cellCspecific proteins with possibly overlapping functions in early hematopoietic progenitors. During embryonic development blood cells arise first in the early yolk sac (primitive hematopoietic cells) and later independently in the vicinity of the dorsal aorta (definitive hematopoietic cells; for reviews see Dzierzak and Medvinsky, 1995; Zon, 1995; Cumano et al., 1996; Dieterlen-Lievre et al., 1996). After the transient production of blood cells in the spleen and fetal liver (mammals), hematopoietic progenitors are produced exclusively in the bone marrow, where their expansion and growth can be controlled by an complex arranged of microenvironmental cues elaborated by stromal cells (Quesenberry, 1992). The evaluation of hematopoiesis offers been significantly facilitated by the id of a range of cytokines (for review discover Callard and Gearing, 1994) and of particular cell surface area antigens (for evaluations discover Spangrude et al., 1991; Uchida et al., 1993) that allow the remoteness and development of monopotent and multipotent precursors. In revenge of their substantial curiosity, antigens known to become indicated on the surface area of hematopoietic come cells are still fairly few. They comprise tyrosine kinase receptors such as c-kit (for review discover Bernstein et al., MK-0679 1991) and flk-2 (Matthews et al., 1991), mucins such as Compact disc34 (Simmons et al., 1992), glycosylphosphatidylinositol-linked substances of unfamiliar function such mainly because Sca-1 and Thy-1 (Uchida et al., 1993; Kilometers et al., MK-0679 1997), and the AA4.1 antigen, a particular gun of yolk sac and fetal liver organ hematopoietic progenitors (Michael jordan et al., 1990). non-e of these guns are definitely particular for hematopoietic come cells and they must become utilized in mixture with lineage-specific markers to separate monopotent from multipotent progenitors (Uchida et al., 1993). In previous work we found that the Myb-Ets oncoprotein-encoding acute leukemia virus E26 is able to transform primitive hematopoietic progenitors derived from chicken embryo yolk sac. These cells resemble multipotent hematopoietic progenitors since they can be induced to differentiate into either erythrocytes, thrombocytes, myeloblasts, or eosinophils and we have therefore designated them as MEPs1 (Myb-EtsCtransformed Progenitors; Graf et al., 1992). Using MEPs as a source of antigen for immunizations we have generated a panel of monoclonal antibodies directed against the surface antigens of these progenitors (McNagny et al., 1992). One of these antibodies, named MK-0679 MEP21, was shown to react specifically with an antigen present on MEPs but absent on transformed B and T lymphoid, erythroid, myelomonocytic, and eosinophilic cell lines. Surprisingly the antigen was also found to be expressed on thrombocytes obtained after differentiation induction (through v-Myb inactivation) of MEPs transformed by a temperature mutant of E26 virus (Frampton et al., 1995). Likewise, the MEP21 antigen could be detected on normal chicken thrombocytes, but not on lymphocytes, erythrocytes, eosinophils, neutrophil granulocytes, or macrophages (Graf et al., 1992; McNagny et al., 1992). For several years, we had attempted to sequence MEP21 by conventional protein chemical techniques. However, these attempts were unsuccessful due to the very low amounts of protein that could be purified (silver stained level). c-COT Here MK-0679 we report the use of nanoelectrospray mass spectrometry (Wilm and Mann, 1996; Wilm et al., 1996) to sequence the MEP21 protein and clone MEP21-encoding cDNAs and a detailed analysis of the expression of the antigen during ontogeny. The data display that MEP21 can be a new mucinlike proteins related to Compact disc34 distantly, which can be indicated on the surface area of mono- and multipotent progenitors of both simple and defined origins..