In order to determine whether and how estrogen regulates AQP3 expression in ER-positive breast cancer cells, we treated three breast cancer cell lines including ER-positive T47D and MCF7 cells and ER-negative MDA-MB-231 cells with estradiol (E2), and found that treatment with 10−8 M and 10−7 M E2 for 48 h significantly upregulated the expression level of AQP3 mRNA in ER-positive breast cancer cells (T47D, Fig. 2A; MCF7, Supplementary Figure S2A), but not in ER-negative breast cancer cells (MDA-MB-231, Fig. 2B). This evidence concerns the gene AQP3 and breast cancer.