AHR and breast cancer: Although it is recognized that several factors contribute to CYP1B1 expression [73,74,75], CYP1B1 levels were chosen as an approximation of AHR activity for three reasons: (1) We and others previously demonstrated that baseline CYP1B1 mRNA levels are maintained to a large extent by “constitutively active” AHR in human breast cancer cell lines [46,76].