To screen the compounds for bile-duct toxicity, Michael Pack’s team at Penn exposed zebrafish larvae to various chemical fractions from Dysphania. In his model, the fish swallowed fluorescent phospholipids so researchers could visualize how they travelled through the digestive system. “If there was a toxin, you wouldn’t see it travel to the gall bladder because the ducts would be blocked,” says Pack. Sure enough, one of the chemical fractions blocked the fluorescent phospholipids. Multiple rounds of subfractionating and screening eventually fingered the culprit: a compound the team dubbed biliatresone (Science Translational Medicine, 7:286ra67, 2015). The chemical did not affect any other tissues of the body.