Regulatory forum opinion piece: Tox21 and toxicologic pathology.

Tox21 is a transformative effort on the part of several U.S. Federal agencies (the National Toxicology Program/National Institute of Environmental Health Sciences [NTP], the National Institutes of Health (NIH) Chemical Genomics Center/National Human Genome Research Institute [NCGC; now called the NIH Center for Translational Therapeutics, National Center for Advancing Translational Sciences] the Environmental Protection Agency's (EPA) National Center for Computational Toxicology, and recently the Food and Drug Administration) that are partnering to fundamentally change the science of safety toxicology. These agencies bring a comprehensive suite of capabilities and are working diligently together to develop, evaluate, and ultimately implement a new safety assessment paradigm. Toxicologic pathology has an important ongoing role in establishing the validity of this transformation, and may ultimately benefit as a discipline through an enhanced understanding of chemically induced disease mechanisms.