We investigated the Cdx2 status in 2 ER-positive (MCF7 and T-47D) and in 8 ER-negative (MDA-MB-231, SUM 159PT, SK-BR-3, BT549, MDA-MB-468, HCC1143, BT20 and HCC1954) human breast cancer cell lines by pyrosequencing analysis. This evidence concerns the gene ESR1 and breast cancer.