Chromatin loop extrusion is a popular model for the formation of CTCF loops and 
topological domains. Recent HiC data have revealed a strong bias in favour of a 
particular arrangement of the CTCF binding motifs that stabilize loops, and 
extrusion is the only model to date which can explain this. However, the model 
requires a motor to generate the loops, and although cohesin is a strong 
candidate for the extruding factor, a suitable motor protein (or a motor 
activity in cohesin itself) has yet to be found. Here we explore a new 
hypothesis: that there is no motor, and thermal motion within the nucleus drives 
extrusion. Using theoretical modelling and computer simulations we ask whether 
such diffusive extrusion could feasibly generate loops. Our simulations uncover 
an interesting ratchet effect (where an osmotic pressure promotes loop growth), 
and suggest, by comparison to recent in vitro and in vivo measurements, that 
diffusive extrusion can in principle generate loops of the size observed in the 
data. Extra View on : C. A. Brackley, J. Johnson, D. Michieletto, A. N. Morozov, 
M. Nicodemi, P. R. Cook, and D. Marenduzzo "Non-equilibrium chromosome looping 
via molecular slip-links", Physical Review Letters 119 138101 (2017).