In addition, the ability of low endoxifen concentrations to significantly inhibit estrogen-induced gene expression and proliferation in ER-β-expressing BC cells suggests that the benefits from tamoxifen therapy may still be observed in patients characterized as poor metabolizers based on their Cytochrome P450 2D6 genotype if their tumors are ER-β-positive [43]. The gene discussed is CYP2D6; the disease is breast cancer.