CD109 and neoplasm: However, endogenous human CD109 could be present in sera of human subjects, which may confound quantitative assessments of tumor-produced CD109 in human serum, as previous publications have reported CD109 expression in activated T-cells, activated platelets, and endothelial cells; this implies that serum CD109 is secreted from hemocytes (including T-cells), platelets or endothelial cells [1]–[7].