However, previous studies have provided evidence that the cellular uptake of estrogens and related hormonal therapeutics used in the treatment of breast cancer, such as tamoxifen, may be regulated by the polymorphic organic anion-transporting polypeptides OATP1B1 (gene name, SLCO1B1) and/or OATP1B3 (gene name, SLCO1B3; refs. 10–12). Here, SLCO1B1 is linked to breast cancer.