Focusing on the subgroup of patients with tamoxifen as their only systemic treatment did not reveal any statistically significant association between CYP2D6 activity and breast cancer recurrence (aHR 1.39, 95% CI 0.99; 1.96) or breast cancer mortality (aHR 1.88, CI 0.98; 3.60). The gene discussed is CYP2D6; the disease is breast cancer.