CD34 and cancer: CD109 is expressed in a subset of haematopoietic cells, bone marrow CD34+ cells, platelets, endothelial, activated T cells and mesenchymal stem cells.16,22 However, it is rarely expressed in normal human tissue, with the exception of bronchial epithelial, mammary myoepithelial cells, lacrimal and salivary glands and prostate basal cells.7,23 As a cancer-associated marker, CD109 was upregulated in various tumour tissues, cancer cell lines and CSCs.