ER positive (MCF-7 subclones (H), (M), (L); T-47-D: low ER expression, 14 fmol/mg [30]) and negative (MDA-MB-231, HCC1806 and HCC1937) breast cancer cell lines were characterized in terms of antiestrogen sensitivity, ER and Y1R expression. This evidence concerns the gene ESR1 and breast cancer.