CD34 (Cluster of differentiation 34) is a single-pass type I transmembrane glycoprotein which primary functions as a cell to cell adhesion factor. As an adhesion factor, CD34 is expressed during stem/progenitor stage of lymphohematopoietic development and possibly mediates the stem cell attachment to the bone marrow, ECM or stromal cells. CD34 is expressed on hematopoietic stem/progenitor cells, endothelial cells, fibroblasts and other stromal components. CD34 is an important marker for quantifying and purifying hematopoietic progenitor/stem cells. It is useful in identification of tumors with endothelial or lymphoid differentiation. In addition, CD34 aids in detection of gastrointestinal stromal tumors.