This is a monoclonal antibody to the A-subunit of human coagulation Factor XIII. Studies have shown it recognizes human Factor XIII Achain in both reduced and non-reduced forms. It does not react with human Factor XIII B-chain or human Factor XII. Factor XIII is a betaglobulin found in plasma and is composed of two subunits. Factor XIIIA is the catalytic subunit and is a dimer of M.W. 160,000. According to studies, Factor XIII is present in plasma as an alpha2beta2 heterodimer (M.W. 320,000); whereas in platelets, only the alpha2 unit exists. Factor XIIIa is a dermal dendrocyte marker and shows variable reaction with these types of tumors (3,4). It can be used for histiocytic phenotyping and has been reported to mark capillary hemangiomas and tumors of the central nervous system (1-6). Factor XIIIa has also been used with CD34 to differentiate between dermatofibroma and dermatofibrosarcoma protuberans (3).