CYP2D6 and breast cancer: The hypothesis stems from a long line of evidence showing that activity impairing polymorphisms in the tamoxifen‐metabolizing enzyme CYP2D6 significantly reduce concentrations of endoxifen and 4OHtam in BC patients [11, 12, 13] and multiple studies have linked CYP2D6 genetic variants with diminished metabolic activity to worse tamoxifen treatment outcome [14].