Chlorpyrifos is weakly estrogenic4,5,52 and antiandrogenic.53,54 It can affect hormone pathways as an aryl hydrocarbon receptor agonist55 and induce proliferation of estrogen-dependent breast cancer cells in vitro.56 At low doses, chlorpyrifos promotes mammary tumor development and alters mammary gland hormone balance in vivo.57,58 More toxicologic research is needed to understand its mechanistic potential with regard to breast cancer risk. The gene discussed is AHR; the disease is breast cancer.