These same principles of predictive toxicology can be used in a regulatory setting for prioritizing chemicals such that those with the greatest potential to produce biological effects can be identified for further evaluation. Chemical testing prioritization for the vast majority of commodity chemicals (e.g., those regulated under the Toxic Substances Control Act in the United States) is based largely, if not exclusively, on production volumes. While this makes some sense, in that production volume is correlated with exposure potential, it is weighted entirely toward the exposure side of risk. This problem has been recognized, but it has been difficult to do anything about it because our imperfect knowledge about mechanisms of toxicity has limited the available tests for toxicity to models that focus on outcome-that is, toxicity in an intact organism.