Many papers have reported that the mRNAs for both ERα and ERβ are expressed in breast cancer cell lines (Watanabe et al, 1997; Moore et al, 1998; Vladusic et al, 2000), in breast cancer tissue (Dotzlaw et al, 1997) and in the normal human and rodent mammary gland (Moore et al, 1998; Saji et al, 2000). This evidence concerns the gene ESR2 and breast cancer.