(A) Gel electrophoretic analysis of HeLa cell DNA fragmentation. At 2 or 4 h after incubation of HeLa cells with Stx1 (100 pg/ml to 100 ng/ml), DNA fragmentation analysis was performed as described in Materials and Methods. Lane 1, control without Stx1; lanes 2 through 11, Stx1 treatment. Cells were treated with Stx1 at 10 pg/ml (lanes 2 and 3), 100 pg/ml (lanes 4 and 5), 1 ng/ml (lanes 6 and 7), 10 ng/ml (lanes 8 and 9), or 100 ng/ml (lanes 10 and 11) and were analyzed at 2 h (lanes 2, 4, 6, 8, and 10) or 4 h (lanes 3, 5, 7, 9, and 11) after treatment. (B) Quantitation of Stx1-induced DNA fragmentation. The percent DNA fragmentation per total cellular DNA was determined by the Burton method to be 5.37% ± 1.75% at 1 h, 21.15% ± 1.73% at 2 h, 51.91% ± 4.43% at 3 h, and 64.62% ± 3.03% at 4 h after treatment of HeLa cells with Stx1 (10 ng/ml).