SMC-mediated chromosome organization: Does loop extrusion explain it all?

In recent years, loop extrusion has attracted much attention as a general mechanism of chromosome organization mediated by structural maintenance of chromosomes (SMC) protein complexes, such as condensin and cohesin. Despite accumulating evidence in support of this mechanism, it is not fully established whether or how loop extrusion operates under physiological conditions, or whether any alternative or additional SMC-mediated mechanisms operate in the cell. In this review, we summarize non-loop extrusion mechanisms proposed in the literature and clarify unresolved issues to further enrich our understanding of how SMC protein complexes work.