Statistical significance of the observed histograms.

Bin sizes within the expected histograms are defined from the variation across the set of wild-type structures: large changes >±2 σ, moderate changes are ±1–2 σ, and no change is between ±1 σ, from which background bin probabilities are calculated. The chi-square statistic is use to compare the expected and observed histograms, and the reported p-values quantify the probability that the histograms are equivalent. In all cases, the histograms are determined to be statistically distinct from the null expectation.