Mammalian cytochromes P450 play a central role in the oxidative biotransformation of exogenous chemicals including drugs, natural plant products, and environmental pollutants. Over the past several decades, P450 2B (CYP2B) enzymes have served as prototypical models for investigation of the mechanism by which drugs and environmental contaminants activate gene expression [1]. CYP2B enzymes are also very versatile catalysts with a broad range of substrates including drugs, environmental pollutants, and steroids [2]. Compared with several other P450 subfamilies, CYP2B enzymes exhibit a relatively low degree of catalytic preservation across mammalian species, making these enzymes an outstanding model system for investigating structure–function relationships of P450s [2] and [3]. Following the discovery of functionally distinct allelic variants of a single rat CYP2B enzyme [4] and the highly structurally related but functionally distinct CYP2B enzymes in rats and rabbits [5] and [6], sequence alignments, mutagenesis approaches, and heterologous expression systems allowed us to identify many key residues that dictate ligand binding orientation within the active site and contribute to substrate and inhibitor specificity of several CYP2B enzymes. The major findings on the structural basis of CYP2B specificity from mutagenesis studies were summarized in a previous review