CD34 and acute myeloid leukemia: Originally identified by mAbs raised against the primitive CD34+ acute myeloid leukaemia cell line KG1a, CD109 is a transforming growth factor (TGF)-β co-receptor that binds TGF-β1, regulates TGF-β receptor endocytosis and degradation, and suppresses TGF-β/Smad signalling [19–23].