Compared to the dramatic increase in CYP1A1 mRNA by TCDD in estrogen receptor (ER)α(+) MCF-7 (human breast adenocarcinoma) cells [6], ERα(-)MDA-MB-231 (MB-231) (human breast adenocarcinoma) cells carry an ARNT variant with an incomplete glutamine-rich region and transactivation domain, resulting in an inefficient CYP1A1 induction [7]. This evidence concerns the gene ESR1 and breast adenocarcinoma.