This study will investigate mast cell precursors that circulate in the blood. In a group of diseases collectively known as mastocytosis, mast cells accumulate in abnormal amounts in the skin, lymphoid tissues, bone marrow, liver and spleen. Some forms of mastocytosis have a generally good prognosis; for others, the prognosis is poorer. There is no known cure for any form of the disease. A better understanding of mast cells and how they respond to certain substances may provide insights that will lead to effective treatments for mastocytosis.