CD34 is a hematopoietic stem cell marker and transmembrane adhesion protein that is also expressed in endothelial cells lining vessels, liver sinusoids and lymphatics, where staining is typically membranous. Cells expressing CD34 (CD34+ cell) are normally found in the umbilical cord and bone marrow as hematopoietic cells, or in 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, as well as cells in soft tissue tumors like DFSP, GIST, SFT, HPC, and to some degree in MPNSTs. It is associated with the selection and enrichment of hematopoietic stem cells for bone marrow transplants. In tumors, CD34 is found in alveolar soft part sarcoma, preB-ALL (positive in 75%), AML (40%), dermatofibrosarcoma protuberans, gastrointestinal stromal tumors, giant cell fibroblastoma, granulocytic sarcoma, Kaposi’s sarcoma, liposarcoma, malignant fibrous histiocytoma, malignant peripheral nerve sheath tumors, mengingeal hemangiopericytomas, meningiomas, neurofibromas, schwannomas, and papillary thyroid carcinomas. References: Sidney, Laura E et al, Stem Cells 2014, 32.6:1380; Hua, 2011; Catteau, 2013, PMID: 24497003;