ATM-deficient cells are resistant to Fas-induced apoptosis. ATM-proficient cells (C3ABR) and ATM-deficient cells (L6) were treated with 250 ng/mL anti-Fas mAb. Apoptosis was determined by the analysis of DNA fragmentation in (A) propidium iodide–stained cells (PI) or by the analysis of (B) annexin V binding, 24 hours after anti-Fas treatment. (C) ATM-deficient cells (L6) were stably transfected with ATM-wt, ATM-Kin⁻, or empty vector as control using 20 μg of the indicated constructs. For immunoblotting, 80 to 100 μg protein extract were separated by SDS-PAGE and transferred on nitrocellulose. ATM protein was revealed with anti-ATM (MAT3) antibodies. (D,E) Cells were treated to undergo apoptosis with 250 ng/mL anti-Fas mAb. Apoptosis was determined by the analysis of DNA fragmentation on PI nuclear staining (D) or by the analysis of annexin V exposure (E) at 24 hours after anti-Fas treatment.