ESR1 (Estrogen Receptor 1) encodes an estrogen receptor that is a ligand-activated transcription factor composed of several domains important for hormone binding, DNA binding and activation of transcription. The protein localizes in the nucleus where it may form a homodimer or a heterodimer with estrogen receptor 2 and affect cellular proliferation and differentiation in target tissues. Estrogen and its receptors are essential for sexual development and reproductive function, but are also responsible for the growth of the skeleton and for maintaining the normal function of the cardiovascular and nervous systems. Estrogen receptors are also involved in pathological processes including breast cancer, endometrial cancer and osteoporosis. Its related pathways include ovarian tumor domain proteases and cell cycle role of SCF complex in cell cycle regulation.