Previous studies have shown that adjuvant tamoxifen users with a PM status (only based on CYP2D6 genotype) have an increased risk of breast cancer recurrence and mortality (Goetz et al, 2005; Lim et al, 2007; Schroth et al, 2007; Bijl et al, 2009). Here, CYP2D6 is linked to breast cancer.