News Finding: They then found that the loss of SMCHD1 prevented the spreading of Xist across the chromosome and kept the two large domains, which the team named S1 and S2 compartments, from being merged into the typical compartment-less inactive X structure.
Paper Finding: We speculate that, when Xist spreads from A to adjoining B compartments, S1 and S2 structures are formed.
News Context: The team's experiments in mouse cells first confirmed that the presence of SMCHD1 was necessary for the silencing of a large subset of genes on the inactive X chromosome. They then found that the loss of SMCHD1 prevented the spreading of Xist across the chromosome and kept the two large domains, which the team named S1 and S2 compartments, from being merged into the typical compartment-less inactive X structure. By bridging the S1 and S2 compartments, SMCHD1 was found to induce their merging into the conformation that prevents gene expression.
Paper Context: Furthermore, S1/S2 compartments sometimes resemble fusions of several consecutive A/B compartments ( Figure 7B , dotted arrows). We speculate that, when Xist spreads from A to adjoining B compartments, S1 and S2 structures are formed. The creation of S1/S2 structures does not require SMCHD1, as SMCHD1 is not enriched on the Xi during this transitional stage.