CD34, which was first detected in hemopoietic and lymphopoietic progenitors, is a heavily glycosylated Type I transmembrane protein that does not share any significant similarity with other transmembrane proteins. Its functions are still unknown. Several monoclonal antibodies were raised against CD34, and at least 4 different epitopes could be recognized. CD34 expression is confined to a few cell lines, to 1-4% of adult bone marrow mononuclear cells (including marrow-repopulating cells, all multipotent and committed myeloid progenitors, B and T lymphoid precursors, osteoclast precursors, and most likely the precursors for stromal cells), and to less than 1% of peripheral blood mononuclear cells. In non-lymphohemopoietic tissues its expression is confined to endothelial cells and to some cells of the skin. In malignancies, CD34 expression is not fully elucidated. Immature hemolymphopoietic malignancies (namely acute leukemias) and the blast cells of chronic myeloid leukemia are frequently positive. Chronic lymphoproliferative disorders and lymphomas are negative. Among other tumors, only vascular derived tumors are positive. Clinical applications of CD34+ cells include autologous transplantation of putative CD34+ stem cells isolated by positive selection from the bone marrow, and transplantation of autologous peripheral blood stem cells, using the proportion and number of CD34+ cells as a guideline for the harvesting procedure.