ALPS patients with somatic mutations carry both Fas-competent and Fas-deficient B cells. (A) Fas-induced apoptosis of EBV-transformed B-cell clones of P1, P2, and a healthy donor (HD). Both patients had B-cell populations sensitive as well as resistant to Fas-induced apoptosis (representative of 2 independent experiments). (B) Surface Fas expression in EBV-transformed B-cell clones of P1 and (C) P2. (D) Schematic depiction of Fas expression (short black line) and function (black flash, Fas-induced apoptosis normal; gray flash, Fas-induced apoptosis defective in P1 and P2).