CYP2D6 and breast cancer: The OR for breast cancer-free interval for tamoxifen compared to letrozole in individuals with any reduced function CYP2D6 allele was 1.05 (95% CI: 0.72 to 1.52), and for individuals with normal function CYP2D6 allele, it was 1.26 (95% CI: 0.94 to 1.69), with no evidence for effect modification (P=0.44) (Table 2).