FAS expression by CD34⁺ cells. BM cells isolated from a patient with PNH (A, C) and a healthy donor (B, D) were analyzed by two-color flow cytometry with FITC-labeled anti-CD34 and PE-labeled anti-FAS MoAbs before (A, B) and after (C, D) treatment with TNF-α and IFN-γ. FAS expression on the gated CD34⁺ cells is shown in the histograms. A second experiment with cells from four other patients with PNH and four healthy donors gave similar results.