Pinpointing proteins To develop drugs that target a specific cell surface protein, it's helpful to know which other proteins reside in its vicinity. Geri et al. report a light-triggered labeling technique that improves the spatial resolution for this type of mapping. Specifically, they rely on a photocatalyst with a very short energy-transfer range to activate a carbene-based label that can only diffuse a short distance in water before reacting. They showcase the technique by mapping the environment of the programmed-death ligand 1 (PDL1) protein on B cell surfaces, a system of considerable interest in cancer immunotherapy. Science , this issue p. 1091