Extensions of the adaptive response are the toxic responses elicited by Ahr activation. Toxicity results from two different ways of Ahr signaling. The first is a side effect of the adaptive response in which the induction of metabolizing enzymes results in the production of toxic metabolites. For example, the polycyclic aromatic hydrocarbon benzo[ a ]pyrene (BaP), a ligand for Ahr, induces its own metabolism and bioactivation to a toxic metabolite via the induction of CYP1A1 and CYP1B1 in several tissues.  The second approach to toxicity is the result of aberrant changes in global gene transcription beyond those observed in the "Ahr gene battery." These global changes in gene expression lead to adverse changes in cellular processes and function.  Microarray analysis has proved most beneficial in understanding and characterizing this response.