The more restricted expression of CD117 compared with CD45 in the hematopoietic compartment enabled the use of a 6-fold lower dose (0.5 mg/kg versus 3.0 mg/kg) of CD117-sap to achieve robust HSC depletion in the absence of prolonged lymphopenia, making it more cost effective as well as limiting exposure of the recipient to the drug-toxin conjugate. The gene discussed is KIT; the disease is lymphopenia.