The proteins in our cells are not solo players; they constantly engage and cooperate with other proteins. These interactions are very important for the normal function of the cell and are relevant to many diseases. The question is - how do you track these protein-protein interactions inside a living cell? In the Muir lab, we are developing tools that allow us to answer this question. Our techniques must have high spatial and temporal resolution so we can accurately identify even the weakest and shortest-lived protein-protein interactions. Thus, we use and develop photo-proximity labelling tools. In these techniques, a photocatalyst is bound to a protein of interest. Upon a short light irradiation, the photocatalyst excites chemical probes to a reactive state, which then tag molecules in their surroundings. In this video I show a colorful step from the synthesis of a photocatalyst and demonstrate how we use visible light to capture a protein’s interactors.