To keep their insides in and their outsides out, cells and their substructures are enveloped by lipid bilayer membranes. The membranes have the consistency not of rubbery balloons but of thin liquid films: Proteins and other molecules embedded in the membranes can diffuse from place to place. But despite that diffusion, the membranes are compositionally heterogeneous, not uniformly mixed, and they sometimes appear to host discrete domains enriched in particular molecules.

Biologists and biophysicists have long debated the mechanism of membrane organization—and, in particular, whether it has anything to do with thermodynamic phase separation, the same physics that drives the demixing of a shaken bottle of vinegar and oil. Now Sarah Keller, Alexey Merz, and their colleagues at the University of Washington in Seattle have shown that membrane domains do indeed form through phase separation, at least in some living cells.1 

Keller’s interest in the problem was from a...

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