The early studies of hematopoiesis were based on tedious in vitro cell culture and in vivo transfer experiments. In 1985, a major advance was made with the discovery of a monoclonal antibody which bound a cell-surface moiety on all hematopoietic progenitors. Subsequently designated as CD34, this molecule remains the only phenotypic marker that identifies early hematopoietic cells. Approximately 0.1% of normal peripheral blood mononuclear cells and 1% to 4% of human bone marrow cells express the CD34 antigen. The identification and characterization of CD34+hematopoietic stem cells (hematopoietic stem cell) has opened new fields of research, and has become amajor interest for both biological and clinical research.