Patient neoplastic mast cells are sensitive to 17-AAG. Bone marrow mononuclear cells isolated from 4 patients with systemic mastocytosis were separately incubated with increasing concentrations of 17-AAG as described in “Patients, materials, and methods.” Mast cell percentage of the total mononuclear fraction is shown. The control group, grown in the absence of SCF, demonstrates that the neoplastic cells in mastocytosis are not dependent on exogenous SCF for survival. Data are expressed as mean; n = 4; ^*P < .05.