Southern blots of BV173 DNA showing drug-induced cleavage of the MLL BCR. (A) The lanes from left to right show the marker (M), a negative control [(−) where no drugs were added to BV173 cells], VP16 cell treatment (100 μmol/L, 200 μmol/L), VM26 cell treatment (50 μmol/L, 100 μmol/L), and doxorubicin (Dox) cell treatment (10 μmol/L). An increasing amount of drug added is indicated by a triangle above the lanes. The PCR genomic probe (tel; see Fig 1) was hybridized to the Southern blot. The 8.3-kb germline DNA fragment and the new 1.5- to 1.6-kb drug-induced DNA fragment are indicated. (B) The lanes from left to right show DNase I–treated nuclei (0 to 20 U as indicated), a 16-hour drug treatment of BV173 cells, including a dimethyl sulfoxide control (C), VP16 (100 μm—two lanes), and N-Methylformamide (NMF; 0.5 mol/L). The PCR cDNA probe indicated above (0.74 kb) was hybridized toBamHI-digested DNA. The 8.3-kb germline and two new DNase I and drug-induced DNA fragments (1.5 kb and 6.9 to 7.0 kb) are indicated to the right. The DNase I, topo II, and DNA damaging agent cleavage sites either map within exon 9 or just telomeric of exon 9. Note that the new 6.9- to 7.0-kb DNA fragment is strongly hybridizing with exons 5, 6, and 7 and either all or half of exon 9. In contrast, the 1.5-kb DNA fragment is hybridizing more weakly with either none or half of exon 9, plus exon 10 and half of exon 11 (133 bp or 206 bp, respectively). The 1-kb marker (M) indicated to the left identifies a few of the molecular weight bands that cross-hybridize with this probe.