The antigen and epitope have not been further characterized. Antigen distribution: Isolated cells: The antigen is found in variable numbers on freshly isolated umbilical vein endothelial cells (HUVECs). Antigen expression on cultured HUVECs is dependent on the culture conditions. Best results were obtained (40-70 % pos.) by daily renewal of FCS containing medium supplemented with endothelial cell conditioned supernatant whereas commercially available endothelial cell media apparently have no influence on the antigen expression. Specificity: Human: continuous endothelial cells. Other: negative in mouse negative, other species not tested.