N-BPs induce DNA double-strand breaks. MCF-7 cells were treated with 10-4M ALE, IBA and RIS for 72 h. As a positive control, the cells were treated with 50 μM etoposide (ETO) for 24 h. Cells were then fixed and stained for γH2A.x (green). Nuclei were visualized by propidium iodide (P.I.) counterstaining (red). Scale bar: 4 or 20 μm.