Northern blot analyses using 32 P-labeled oligonucleotide probes (Table 1) for 18S and 28S rRNAs were performed to identify and map sites of rRNA cleavage. Incubation with five probes complementary to 28S rRNA revealed six fragments (a-f) with sizes approximating 4000, 3200, 2800, 1. Detection of DON-induced rRNA cleavage in RAW 264.7 by agarose gel and capillary electrophoresis. Cells were treated with or without 1000 ng/ml DON for 6 h. RNAs were purified and analyzed either (A) on 1.2% formaldehyde denaturing agarose gel (10 lg) or (B and C) by capillary electrophoresis (300 ng). The x-axis indicates the size of the fragments in nucleotide (nts) and the y-axis indicates the relative peak intensity in fluorescence units (FU). The two major peaks represent 18S rRNA (~2000 nts) and 28S rRNA (~4000 nts). Arrows designate three significant cleavage peaks between 28S and 18S rRNA and two peaks below 18S rRNA. Rectangle in C indicates chart region to be shown in subsequent figures. 384 HE, ZHOU, AND PESTKA 1500, 1000, and 500 nts (Fig. 3A) . Based on rRNA fragment sizes, probe position and Northern blotting, putative cleavage sites of 28S rRNA were deduced ( Figure 3B ). DON appeared to cleave 28S rRNA into one of two pairs of fragments: a þ e (approximately 5000 nts) and b þ d (approximately 4700 nts). Because no complementary fragment (approximately 2000 nts) to fragment c was detected, it was likely to be a product of the subsequent degradation of a or b.