ESR1 and breast cancer: We previously demonstrated that the resistance of breast cancer to antiestrogens is mediated through both increased proteasomal ER degradation and increased transcriptional activation of HER2 by YBX1 [26] and that YBX1 negatively and positively correlates with ER and HER2 expression, respectively, in clinical specimens from breast cancer patients as well as in breast cancer cells in vitro and in vivo [21, 26].