In the new study, researchers designed a virus to send an enzyme to a precise location in the brain of a living mouse. Derived from soybeans, the enzyme genetically tags its neighboring proteins in a predetermined location. After validating the technique by imaging the brain with fluorescence and electron microscopy, the researchers found their technique took a snapshot of the entire set of proteins (or proteome) inside living neurons, which can then be analyzed postmortem with mass spectroscopy.