ESR1, also known as Estrogen Receptor Alpha, is a protein that belongs to the nuclear hormone receptor family and acts as a transcription factor. It plays a crucial role in mediating the effects of estrogen, a steroid hormone, across different tissues in the body. ESR1 is vital for regulating gene expression in response to estrogen binding. In the absence of estrogen, ESR1 stays in the cytoplasm in an inactive state, attached to heat shock proteins. When estrogen binds, ESR1 undergoes a conformational change, detaches from the heat shock proteins, and moves to the nucleus to regulate gene transcription.