or X-Z interactions. More concretely, the abundant interactions occurring throughout a megabase chromosomal domain such as a TAD/CID can be conceptualized as a globular domain in which all interacting pairs of chromosome fragments are in close proximity in a given cell, facilitating many interactions. Alternatively, a TAD/CID could also result from an ensemble population of individual cells with more extended chromatin in which a given cell might exhibit a single looping interaction within the domain of interest, but with many different loop structures occurring throughout the cell population. Initial results with single-cell Hi-C (Nagano et al. 2013) show a large amount of variability in terms of large-scale structure, although this is somewhat hard to interpret physically given the small number of interactions identified for any individual cell in this assay. Future studies should help reveal the single-cell details of such multilocus interactions.
