Fig. 4.
The SBS model captures the full range of values of the distance kurtosis observed in FISH data. (A) The ratio of the fourth and second moment of the distance R between loci at the genomic distance s [i.e., the kurtosis; 
] is plotted as a function of s. It provides a measure of the relative amplitude of fluctuations of the polymer conformations. K = 1.50 when R2(s) is randomly distributed as a self-avoiding polymer (horizontal dashed line). Experimental K values depart from 1.50; K values were first analyzed in ref. 35, and originate from human fibroblast chromosome 1 ridges or whole chromosomes 1 or 11 (9) (squares, open circles, and filled circles, respectively), and from pre/pro-B or pro-B cell murine immunoglobulin heavy chain locus (10) (light- or dark-blue diamonds, respectively). (B) The kurtosis measured in the SBS model is plotted as a function of cm. K is close to 1.5 at low and high concentrations of binding molecules (open and closed chromatin). Around the binder threshold concentration, K exhibits a peak with values up to approximately 5. The range of values of K measured experimentally (A) matches the range found within the SBS model. It emerges that, beyond open and compact states, chromatin loci are likely to include also fractal conformations corresponding to the transition point.