Estrogen receptor α(ERα) plays an important role in breast cancer development and progression and thus becomes a useful molecular target for breast cancer therapy. ERα is differentially expressed in breast cancer patients. Moreover, ERα expression levels may change at different stages of breast cancer even for the same patient. ERα expression is closely associated with the effect of endocrine therapy and prognosis. The mechanisms underlying ERα expression are complicated, because ERα expression

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... is regulated at different levels, including chromatin, transcriptional, post-transcriptional, translational, and post-translational levels. Many proteins modulate the transcription of ERα gene at the chromatin and transcriptional levels through direct or indirect interaction with the ERα promoter. Some microRNAs decrease ERα levels possibly by induction of the degradation of ERα mRNA and/or repression of the mRNA translation. At the post-translational level, many proteins regulate ERα protein levels through ubiquitin-proteosome pathway. This review focuses on molecular mechanisms of regulation of ERα expression at different levels.