Stem cells are cells produced by the bone marrow that mature into the different blood components-white cells, red cells and platelets. Transplantation of allogeneic (donated) stem cells is a mainstay of therapy for some forms of leukemia. Patients first receive intensive chemotherapy and radiation to rid the body of cancer cells. This "conditioning" is followed by transplantation of donated stem cells to generate new, healthy bone marrow. In addition to producing the new bone marrow, the donated cells also fight any residual tumor cells that might have remained in the body. This is called a "graft-versus-tumor" effect. This study will examine whether a stem cell transplant from a healthy donor can similarly target and destroy mast cells in a "graft-versus-mast cell" effect. Also, to try to reduce the harmful side effects of chemotherapy and radiation, this study will use lower dose chemotherapy and no radiation.