Overview of isochores on 100 Mb of chromosome 1. The isochores identified on the telomeric 100-Mb region of the short arm of chromosome 1 (as a representative region of human chromosomes) are shown. Broken horizontal guidelines in the top frames represent GC levels as in Figure 1. Horizontal red stretches in the bottom frames represent isochores. Our strategy was to identify boundaries on the basis of GC jumps between adjacent isochores (ΔGC) (see Fig. 3 and Supplemental Table S1). This strategy may occasionally lead to border misassignments, for instance, in the small number of cases (<3%) when ΔGC is lower than 1% (see Fig. 3). Several of the latter borders could be assigned, however, based on differences in the standard deviations of GC (see Table 1 for an example) or in the GC profiles.
