MM6 DNA Southern blot showing the MLL DNase I HS site on the normal 11 chromosomes. Southern blot representingEcoRI-digested DNA from DNase I–treated (DNase I units used indicated directly above panels) MM6 whole nuclei was hybridized independently with three DNA probes (indicated above each panel). The 1-kb plasmid marker is shown to the left of (A) and also correlates with hybridizations in (B) and (C). (A) The top 6.0-kb DNA band hybridizing with the MLL 0.6-kb P probe represents the der (9) and is observed in all DNA lanes. The middle MLL 4.5-kb germline (G) DNA band is seen in all DNA lanes. A new 3.2-kbEcoRI/DNase I DNA fragment is only observed in DNase I–treated whole nuclei (arrow). (B) The top 6.0-kb DNA band hybridizing with theMLL 0.9-kb HindIII/EcoRI DNA probe represents the der(9) and is observed in all DNA lanes. The middle MLL4.5-kb germline (G) DNA band is seen in all DNA lanes. A new 3.2-kbEcoRI/DNase I DNA fragment is only observed in DNase I–treated whole nuclei (arrow). This is the same DNA fragment to which the 0.6-kb P probe hybridizes. (C) The top 6.0-kb DNA band hybridizing with theAF9MM6 probe represents the der (9) and is observed in all DNA lanes. The 3.4-kb DNA band represents the AF9 germline (G) region containing the MM6 deletion breakpoint region and is also observed in all DNA lanes.