Apoptosis of CD34⁺ cells from PNH-BM. BM hematopoietic progenitors enriched with CD34⁺ cells obtained from a PNH patient (A, C) and a healthy volunteer (B, D) were analyzed by two-color flow cytometry with both 7-AAD and FITC-conjugated anti-CD34 MoAb before (A, B) and after (C, D) treatment with TNF-α, IFN-γ, and subsequent anti-FAS MoAb. Scattergrams of gated CD34⁺ cells were generated by combining forward light scatter (FSC) with 7-AAD fluorescence and show three regions drawn around clear-cut populations having negative (R1, viable cells), dim (R2, apoptotic cells), and bright fluorescence (R3, late-apoptotic or dead cells), as reported originally by Philpott et al.31 A second experiment with cells from four other patients with PNH and four healthy donors gave similar results.