Restriction map of the MLL BCR showing the location of the MLL probes used in this study, the in vivo topo II cleavage site, and the DNA damaging agent cleavage site (large black arrow above the map). Above the map and indicated as grey hatched boxes are the genomic probes from left to right: the 0.32-kb Rsa I DNA fragment, the 0.8-kb Nco I/BamHI DNA fragment, the 0.6-kb P DNA fragment, and the 0.9-kb H/E DNA fragment. Below the map are the PCR probes: the cen 0.48-kb DNA fragment, the 1.2-kbBgl II/Sac I DNA fragment, the 0.74-kb cDNA, and the tel 0.385-kb DNA fragment. Restriction enzyme sites are indicated along the black line showing the MLL BCR and regions centromeric and telomeric to the BCR. BamHI (B) DNA fragments covering 42 kb (black lines indicated above the map), including the centromeric 24-kb fragment, the 8.3-kb BCR DNA fragment, and the 14-kb telomeric DNA fragment, were analyzed for in vivo cleavage of topo II by hybridizing the 0.32-kb Rsa I, the 0.74-kb cDNA, the tel, and the 0.6-kb P probes to BamHI digestions of etoposide-treated cells. Only the 8.3-kb BCR DNA fragment in BV173 cells was analyzed for cleavage using the DNA damaging agents aclarubicin and N-Methylformamide. Black bars below the map are the weak centromeric and strong telomeric SARs.30 Numbered exons are represented as grey hatched rectangles on the map. The restriction map is not drawn to scale centromeric or telomeric to the double thick black diagonal lines (//). Restriction enzymes are noted on the map as follows: B, BamHI; S, Sac I; H,HindIII; E, EcoRI; Bg, Bgl II; X, XbaI. Note that the Bg* enzyme restriction site is polymorphic in BV173 cells.