Cluster of differentiation 34 (CD34) 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 and progenitor cells, vascular endothelium, embryonic fibroblasts, and rare glial cells in nervous tissue. CD34 is an important marker for quantifying and purifying hematopoietic progenitor/stem cells. CD34 is the most used marker for characterizing blasts in leukemia. CD34 is also present in some soft tissue tumors including solitary fibrous tumors and gastrointestinal stromal tumors. Proliferating endothelial cells seem to upregulate CD34 expression. Although specificity is low, Anti-CD34 reacts positively with more than 85% of angiosarcoma and Kaposi's sarcoma.