Membrane scission by ESCRTs The ESCRT protein complexes are essential for cell division, the release of HIV from infected cells via budding, and other cell processes involving the scission of narrow membrane necks from their inner surface. The unusual inside-directed membrane cutting has made it hard to recapitulate this reaction and understand its mechanism. Schöneberg et al. encapsulated ESCRTs inside lipid vesicles and used optical tweezers to pull out membrane nanotubes. In the presence of adenosine triphosphate, clusters of ESCRTs generated force and constricted the nanotube, eventually severing it. This approach provides a window into the molecular mechanisms involved in the activities of ESCRTs. Science , this issue p. 1423