CYP1B1 and neoplasm: In addition, a recent study has shown that constitutively active AhR contributes to basal CYP1B1 but not CYP1A1 mRNA levels in immortalized and malignant mammary cell lines, whereas AhR hyperactivation by TCDD activates both genes, which implies a contribution of AhR and CYP1B1 prior to tumor formation [50].