Epigenomic assays capture snapshots of complex cellular states. Unlike the DNA sequence, which is relatively stable, chromatin can be highly dynamic, raising the question of the relevant time-scales that govern chromatin structures. This question is related to, but distinct from, the question of heterogeneity within a cell population. When we observe a “fuzzy” nucleosome in an ensemble measurement, does the location of the nucleosome change only at cell division (stable) or every few seconds (dynamic)? A wide range of time-scales is consistent with both the ensemble observation as well as the distribution of single-cell states. This question applies throughout the range of structures we review above, from long-range chromosomal interactions to nucleosome chemical modifications. Understanding the function of the structures observed will ultimately require knowing the relevant time-scales for the structures in question.
