Rules of the road for loop-extruding motors: loading, unloading, and crowding

Loop extrusion by structural maintenance of chromosome (SMC) complexes has emerged as a key mechanism of genome organization across domains of life, yet the rules that govern where these motors load, unload, and become enriched remain poorly understood. The canonical model treats cohesin as loading and unloading uniformly but stopping at extrusion barriers. Chromosome architecture is then shaped by where extruding complexes stop. Accumulating evidence broadens this view by uncovering cases of (i) targeted SMC loading across metazoa, plants, and fungi, (ii) sites of specific SMC release, and (iii) domains of crowded loop extrusion. Together, these new Rules of the Road shift the focus from where extruders stop to where they start, end, and dwell-with implications for development, gene regulation, and genome stability.