BV173 Southern blot showing the MLL BCR DNase I HS site. A Southern blot representing BamHI/BglII-digested DNA from DNase I–treated (DNase I units directly above each lane) and BV173 whole nuclei was hybridized independently withMLL BCR DNA probes (indicated above each panel). λ PhageHindIII/EcoRI-digested marker (left of [A]) correlates with all three hybridizations. (A) Top germline 6.6-kbBgl II DNA fragment hybridizing with the 1.2-kb BglII/Sac I DNA probe is seen in all lanes. A new 5.0-kbBgl II/DNase I DNA fragment (arrow) is not observed in the absence of DNase I, but increases in intensity at higher DNase I concentrations. (B) A germline 2.2-kb DNA fragment is observed in all DNA lanes; thus, the centromeric portion of the MLL BCR is negative for DNase I HS sites. (C) The germline 6.0-kb Bgl II DNA fragment hybridizing with the 0.8-kb Nco I-BamHI DNA probe is seen in all of the DNA lanes. A new 1.4-kb Bgl II DNA fragment is not observed in the absence of DNase I, but increases in intensity at higher concentrations of DNase I (arrow).