CYP2D6 and breast cancer: For breast cancer-specific mortality, CYP2D6*4 heterozygotes and homozygotes had increased risk by 3.7-fold (HR: 3.7, 95% CI: 1.32–10.6, P = 0.01) and 11.6-fold (HR: 11.6, 95% CI: 1.3–103.5, P = 0.03), respectively.