Factor XIIIa (F13A1, Coagulation Factor XIII A subunit) is a blood-clotting transglutaminase that plays an important role in blood coagulation and regulation of insulin signaling. Factor XIIIa participates in wound healing by supporting platelet adhesion and endothelial architecture via cross-links with newly formed fibrin fibers. Consequently, deficiencies in Factor XIIIa are associated with hemorrhage and chronic wounds. In immunohistochemistry of normal tissue, Factor XIIIa is a fibrohistiocytic marker that stains positively in dermal dendrocytes, megakaryocytes, platelets and histiocytic or mesenchymal cells of the prostate, uterus and placenta. It is also positive in macrophages and monocytes. In cancer and disease, Factor XIIIa is positive in benign and malignant fibrous histiocytomas, fibrous tumors, aggressive angiomyxomas, hepatocellular carcinomas, hemangioendotheliomas, capillary hemangioblastomas, hemangiopericytomas, xanthogranulomas, glomus tumors, meningiomas, and also in Erdheim-Chester disease. References: Curr Pharm Des. 2016;22(11):1449-59, PMID: 26654441; Scientific Reports. 2016 Oct. Volume 6, Article number: 35574, DOI: 10.1038/srep35574; Semin Thromb Hemost. 1996;22(5):399-408, PMID: 8989823; Cell Mol Life Sci. 2003 Jun;60(6):1049-60, PMID: 12861374