CD117 staining is shown. CD117, also known as c-kit, steel factor receptor and stem cell factor receptor, encodes a 145 kD cell surface glycoprotein belonging to the class III receptor tyrosine kinase family. It is expressed on the majority of hematopoietic progenitor cells including multipotent hematopoietic stem cells as well as committed myeloid, erythroid, and lymphoid precursor cells. In addition to the potential for the differentiation of hematopoietic cells, CD117+ stem cells from murine bone marrow were reported to be capable of differentiation into smooth muscle cells, myocytes, and endothelial cells in vivo.1,2 CD117 is also expressed on few mature hematopoietic cells, e.g. mast cells. The sections were also richly populated with CD34+ cells (not shown). For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article at www.liebertonline.com/ars.