Among the popular drugs studied in ER+ BC, tamoxifen is among one of the most widely used choice of available treatments and is metabolized to its active forms 4-hydroxytamoxifen (4-OH tamoxifen) and N-hydoxy desmethyl tamoxifen (N-OH desmethyl tamoxifen), which is further metabolized to endoxifen in the liver through the polymorphic cytochrome P450 enzymes (CYP2B6, CYP2C9, CYP2C19, CYP2D6, CYP3A4, CYP3A5), the most effective metabolic enzyme being CYP2D6 (405, 406). The gene discussed is CYP2D6; the disease is breast cancer.