Markers and targets expressed on neoplastic stem cells and mast cells in patients with SM. Neoplastic stem cells propagate the malignancy through their self-renewal capacity and ability to undergo asymmetrical cell division, resulting in daughter cells that are committed to differentiate into neoplastic mast cells (arrows). In common with normal stem cells, neoplastic stem cells in SM display CD34 and KIT as well as CD133. In addition, these cells usually express the cell-surface targets CD33, CD44, and CD52; however, they do not express CD2, CD25, or CD30. Some of the markers used to define stem cells or mast cells, such as KIT, also serve as molecular targets of therapy. Neoplastic mast cells themselves usually display CD2, CD25, and CD30 in an aberrant manner together with KIT, but lack CD34. In addition, neoplastic mast cells in SM express a number of potential therapeutic cell-surface targets, including CD30, CD33, CD52, or CD123.