CYP2D6 and breast cancer: Several studies have shown that the CYP2D6 enzymatic activity in breast cancer patients with CYP2D6*3(775delA), CYP2D6*4(G506-1A), CYP2D6*5 (fragment deletion), CYP2D6*10(C188T), and CYP2D6*41 (c.985+39G>A) alleles is reduced; the levels of activated intermediate metabolites 4-hydroxytamoxifen and endoxifen are decreased after TAM treatment; and the recurrence rate of breast cancer is higher, while the survival rate after recurrence is lower (16–19).