The teachings relates to the three-dimensional structure of a crystal of a cytochrome protein complexed with a ligand. The three-dimensional structure of four cytochrome P450 2A6-ligand complexes are disclosed. Cytochrome P450 2A6-ligand crystal structures, wherein the ligand is an inhibitor molecule, are useful for providing structural information that may be integrated into drug screening and drug design processes. Thus, the teachings also relate to methods for utilizing a crystal structure of a cytochrome P450 2A6-ligand complex for identifying, designing, selecting, or testing inhibitors of the cytochrome protein. Such inhibitors are useful as therapeutics for the treatment or modulation of i) diseases; ii) disease symptoms; or iii) the effect of other physiological events mediated by the cytochrome.