In a living organism, the fusion—as well as the separation, or fission—of membranes happens constantly: when a sperm fertilizes an egg, when HIV enters an immune cell, when neurons release neurotransmitter. Yet the molecular details of this vital process are hard to nail down. Membrane fusion is fast, and scientists can't freeze membranes in the act of merging. They can, however, take a step back and look at links between membrane traffic and human diseases.

Illustration: Emily Forgot