Identifying off-target effects and hidden phenotypes of drugs in human cells. MacDonald et al. (p 329) develop a strategy to directly probe biochemical pathways that underlie therapeutic or toxic mechanisms in intact, living cells. The authors used high-content protein-fragment complementation assays (PCAs) based on fluorescent proteins to measure spatial and temporal changes in protein complexes in response to drugs that perturb specific pathways. They observed known structure-function relationships and deduced 'hidden', antiproliferative activities for four drugs. This study suggests a strategy for discovering unpredicted mechanisms of drug action (see also News and Views by Abraham, p 295). Cover art by Erin Boyle, based on an image of human HEK 293 cells and fluorescence arising from complexes of regulators of G protein signaling (RGS2) and the Frizzled 4 receptor, provided by Jane Lamerdin.