Since tamoxifen functions largely as agonist in molecular level and could recapitulate the gene expression profile induced in breast cancer cells by estrogen [18], here we hypothesized that UBASH3B expression is negatively regulated by tamoxifen through activating ER signaling pathway in tamoxifen sensitive patients while not in tamoxifen refractory patients, which can perfectly explain the correlation between UBASH3B expression and tamoxifen efficacy. This evidence concerns the gene ESR1 and breast cancer.