CYP2D6 and breast cancer: The clinical response to tamoxifen may be connected to its conversion into the active metabolites, Z-4-hydroxy-N-desmethyl-tamoxifen (endoxifen) and Z-4-hydroxy-tamoxifen (4-OH-tamoxifen) by cytochrome P450 2D6 (CYP2D6), as they are 30–100 fold more potent than tamoxifen itself in inhibiting the oestrogen receptor (ER), and the serum concentrations of these metabolites predict long-term survival in breast cancer patients [27].