Although a more reliable direct cellular uptake assay overcoming the nonspecific extracellular membrane binding is warranted in the future, this finding corroborates previously reported data from retrospective studies performed in women with breast cancer receiving treatment with exemestane (8) or letrozole (10) for whom functional OATP1B1 variants were associated with altered estrogen levels and circulating AI concentrations. This evidence concerns the gene SLCO1B1 and breast cancer.