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Factor XIIIa is a blood proenzyme that catalyzes intermolecular cross-linking of fibrinogen and is involved in clotting cascade. It has been identified in various tissues including placenta, uterus, and prostate. Anti- Factor XIIIa has been found to be useful in differentiating between dermatofibroma (90% positive) and desmoplastic malignant melanoma (negative). Factor XIIIa positivity is also seen in capillary hemagioblastoma, hemangioendothelioma, hemangiopericytoma, xanthogranuloma, xanthoma, hepatocellular carcinoma, glomus tumor , and meningioma.