Both major breakpoint regions of FRA3B contain contiguously phased nucleosomes. Nuclei were isolated from aphidicolin-treated and untreated cells and incubated with MNase at 0.00375, 0.0125, 0.0375, and 0.125 U/mL at 37°C for 5 min. Reactions were stopped and DNA was extracted. (A) The Southern blot probing strategy was the same as described in Fig. 3. For probing the FRA3B distal breakpoint region, MNase-treated DNA was further digested with EcoRI and AflIII and hybridized with the D8/D9 probe (left) and the C/I probe (right). The same blot was hybridized with the DHFR probe (data not shown) (B) For probing the proximal breakpoint region, MNase-treated DNA was further digested with EcoRI and hybridized with the 17/18 probe. The same blot was hybridized with the DHFR probe to examine the DHFR promoter region. Symbols are the same as in Figure 3.