Cells exposed to oxidative stress or electrophilic xenobiotics respond by transcriptionally up-regulating a battery of genes that contain a cis-acting element in their promoter region known as the antioxidant/electrophile response element (ARE). Mutational analysis of the promoter regions of ARE-containing genes led to the creation of two different models for the ARE; a core ARE (cARE: RTGACnnnGC) and an extended ARE (eARE: TMAnnRTGAYnnnGCAwwww). Using bioinformatic software we have aligned the promoter regions of several ARE-containing genes to produce two position-specific probability matrices that independently describe the cARE and eARE. These matrices can also be used to quantitatively assess putative AREs.