AHR and breast cancer: Furthermore, notwithstanding habitual measurement of CYP1A1 regulation, a downstream target gene, to confirm the AhR functionality, Lo et al. (2017) found that berberine, a protoberberine alkaloid present in several medicinal plants, such as goldenseal (Hydrastis canadensis), activates AhR but, on the other hand, suppresses CYP1A1 protein expression through the miR-21-3p stimulation in MCF-7 breast cancer cells, which might be a feedback control.