Scientist have utilized so-called SNAP-tag technology to radioactively label cells in living organisms. In a proof-of-principle study they developed a SNAP-tag substrate equipped with the radioactive signal emitter fluorine-18 and used it to make tumor cells in the bodies of mice visible in PET images. The labeling method, already established in microscopy, opens up the prospect of studying cells with different imaging techniques and at different temporal stages — for example, when inflammation begins, continues and resolves again. This may help reveal more about how the functions of individual cells and entire organs are interconnected.