ESR1 and breast cancer: Regarding potential mechanisms whereby FOXA1 regulates HER2 expression, FOXA1 has been shown to regulate HER2 expression by either CREB1 and c-Fos regulation of FOXA1 transcription or AP2α-dependent regulation of both FOXA1 and HER2 expression [21] in molecular apocrine breast cancer cells (which are estrogen receptor- and progesterone receptor-negative but have high expression of androgen receptors and FOXA1) [48,49].