High cell density induces apoptosis and subsequent cleavage of the MLL bcr. (A) Ethidium bromide-stained agarose gel. SUNE1 (lanes 1-3) and HONE1 (lanes 4-6) seeded at cell number of 0.4 × 105 (lanes 1 and 4), 2 × 105 (lanes 2 and 5) and 4 × 105 (lanes 3 and 6) were harvested for genomic DNA extraction after 4 days of growth. DNA was digested with Bam H I and analyzed on 1% agarose gel. M represents the 1 kb DNA marker. (B) A schematic diagram illustrating the 8.3 kb MLL breakpoint cluster region (bcr). B represents the Bam H I restriction site. Black box indicates the position of the DNA probe and down arrow shows the anticipated site of DNA cleavage. (C) Southern hybridization analysis. Southern hybridization was performed using the DNA probe shown in (B). Arrows labeled 8.3 kb and 1.5 kb show the positions of the intact and the cleaved MLL bcr respectively. MDig represents the DIG-labeled DNA marker (Roche, Penzberg, Germany).