Accelerated spontaneous apoptosis of hematopoietic progenitor cells in Kostmann syndrome. (A) The degree of spontaneous apoptosis upon overnight incubation of CD34⁺ early progenitors, CD33⁺ myeloid progenitors, and CD15⁺ neutrophil precursor cells obtained from patient 3. The percentage of apoptotic cells, defined as annexin V⁺, propidium iodide-events, is indicated in the lower right quadrant of each subpanel. (B) Spontaneous apoptosis of CD34⁺, CD33⁺, and CD15⁺ progenitor cell subpopulations from Kostmann patients and healthy controls (mean ± SD; n = 8), as evidenced by annexin V labeling. G-CSF therapy was discontinued for 7 days in patient 3, and for 36 hours in patients 1 and 4, prior to assessment of apoptosis.