Factor XIII is a betaglobulin found in plasma and is composed of two subunits. Factor XIIIA is the catalytic subunit. Factor XIIIa is expressed in platelets, megakaryocytes, fibroblast-like cells in the placenta, uterus, and prostate, monocytes and macrophages and dermal dendritic cells. It is most commonly recognized as a marker of fibrohistiocytic proliferations. Factor XIIIa has also been used with CD34 to differentiate between dermatofibroma and dermatofibrosarcoma protuberans.