THY1 (CD90) is a heavily N-glycosylated, glycophosphatidylinositol (GPI) anchored conserved cell surface protein with a single V-like immunoglobulin domain, originally discovered as a thymocyte antigen. THY1 is thought to be involved in cognition, axon growth regulation, apoptosis, inflammation, fibrosis, tumor suppression, and cell adhesion. THY1 is a useful stem cell marker in immunohistochemistry; pluripotent hematopoietic stem cells (HSCs) show concordant THY1 and CD34 positivity, and THY1 is also expressed in a number of other stem cell types including mesenchymal, hepatic, keratinocyte and endometrial progenitor stem cells. It can be used to illustrate the axonal processes of mature neurons. In normal tissues, Thy1 expression can be found in neuronal cell processes, nerves, immature hematopoietic stem cells, connective tissue, and the kidney. References: Br. J. Dermatol. 2006. 154 (6): 1062–70, PMID: 16704635; J. Exp. Med. 177 (5): 1331–42, PMID: 7683034; Am. J. Physiol. Gastrointest. Liver Physiol. 2006. 291 (1): G45–54, PMID: 16769813;