Apoptosis measurement by acridine orange/ethidium bromide staining. Acridine orange/ethidium bromide (AO/EB) staining was carried out using an adaptation of a previously described procedure (Muppidi et al., 2004) . Briefly, slides were cleaned and sterilized by UV light, placed into 100-mm tissue culture plates, and cultured with RAW 264.7 cells (2.5 3 10 6 ) for 24 h to achieve approximately 80% confluency. DON-exposed RAW 264.7 cells were stained for 2 min with 100 lg/ml AO and 100 lg/ml EB in PBS. The slides were washed twice with cold PBS and then covered with coverslip and examined at 3400 under Nikon fluorescence microscope equipped with a wide-band fluorescein isothiocyanate filter. Cells ( 200) were classified based on their nuclear morphology (bright chromatin, highly condensed, or fragmented nuclei) in to four categories: viable normal (VN), DON-INDUCED RRNA CLEAVAGE viable apoptotic (VA), nonviable apoptotic (NVA), and nonviable necrotic (NVN). The apoptotic index was calculated as follows: