CD34 is expressed on hematopoietic cells, mesenchymal stem cells, endothelial progenitor cells, endothelial cells of blood vessels but not lymphatics (except pleural lymphatics), mast cells, a sub-population of dendritic cells, and in the dermis of skin. CD34 is expressed by a variety of tumors including preB-ALL, AML, GIST, meningiomas, neurofibromas, schwannomas, and papillary thyroid carcinoma.