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According to most biology textbooks, the main organizing principle of the cell is the membrane. Lipid bilayers wrap organelles including the nucleus, mitochondria and the endoplasmic reticulum to keep some proteins in and others out. The rest of a cell’s internal machinery has been depicted as floating awash in the cytoplasm, with proteins occasionally running into binding partners, substrates and small-molecule drugs. Now, a growing appreciation for the importance of biomolecular condensates — transient liquid-like droplets made up of proteins and RNA — is forcing cell biologists to rethink this model.