Book excerpt: This glossary expands upon the Comprehensive Glossary of Terms Used in Toxicology. Like its predecessor, it addresses the need for harmonized toxicology terminology, and has undergone a full, comprehensive and rigorous review by IUPAC Committees. It serves as the reference glossary for students and researchers studying the chemical and molecular biological bases of toxicology, and for those involved in chemicals legislation, regulation and risk assessment. Toxicology uses terminology from many disciplines that have been revolutionized by recent developments in molecular biology and biochemistry, and this glossary provides a compilation of definitions of molecular entities that impact on toxicology. Extensive Notes are included to provide context for many of the proteins and important genes defined, with disambiguation where entities are referred to by different names in the literature. We hope that this will be helpful to students and professionals alike.