The antigen or epitope have not been characterized. Antigen distribution: Isolated cells: The antigen is found in the cytoplasm of cultured endothelial cells of all organs, freshly isolated thrombocytes and megakaryocytes. The antigen expression decreases with increasing number of cell passages. Tissue sections: Endothelial cells in healthy tissues of all organs except small capillaries. The antigen expression is more pronounced in venous than in arterial vessel types. Capillaries of the lung and kidney are negative. The antigen expression decreases in tumorous tissues. Specificity: Human: endothelial cells. Other: bovine and porcine have been reported to be positive.