The W7C5 monoclonal antibody recognizes human CD109 also known as the Gov platelet alloantigen. CD109 is a member of the alpha2-macroglobulin/complement gene family that is a GPI-linked cell surface antigen with a predicted molecular weight approximately 162 kD. CD109 contains a thioester motif characteristic of alpha-2-macroglobulin that undergoes autolytic cleavage under denaturing conditions. CD109 expression has been reported on CD34+ and CD34- bone marrow stem cells, CD34+ cells in the fetal liver, CD34+ acute myeloid leukemia cells, T cell lines, activated T lymphoblasts, activated platelets, and endothelial cells. It is also widely expressed in the brain, uterus, heart, lung, and trachea of adults. The function of CD109 is largely unknown although it has been implicated in refractoriness to platelet transfusion, neonatal alloimmune thrombocytopenia, and post-transfusion purpura.