The FRA3B distal breakpoint region does not have a condensed chromatin structure. Nuclei isolated from aphidicolin-treated (lanes 3,4,13,14), untreated GM 13069 cells (lanes 5,6,15,16) or from FRAXA cells (lanes 7,8,17,18) were incubated with 25 or 100 U of AflIII (indicated by triangles), and purified DNA was completely digested with EcoRI. Genomic DNA from the GM 13069 and FRAXA cells was completely digested with EcoRI (lanes 1,9,11,19) or EcoRI and AflIII (lanes 2,10,12,20). Southern blot analysis was carried out first with the FRA3B distal region specific probe (the C/I probe, top), then reprobing with the W/X probe, specific for the FRAXA site (bottom). The EcoRI fragment of the FRAXA site is 5.2 kb in GM13069 cells, whereas due to CGG expansion, it is 6.8 kb in FRAXA expressing cells. The positions of the distal breakpoint cluster of the FRA3B site and the breakpoint region (the CGG repeating sequence) of the FRAXA site are indicated by blasts. (E) EcoRI; (A) AflIII.