AHR and breast cancer: Use of Cyp1b1 as a marker for AHR activity in this context is supported by (1) our findings [15], and those of others [67], demonstrating that baseline Cyp1b1 mRNA levels are maintained in part by ‘constitutively active’ AHR in human breast cancer cell lines, and (2) the observation that, of all breast cancer cell lines in the CCLE, the nearest neighbor to Ahr of >20,000 gene probes is Cyp1b1 (P = 0.0019; this is not to say that there are no other factors regulating Cyp1b1 expression [67]).