ERα, or Estrogen Receptor Alpha, is a protein that plays a crucial role in the body's response to the hormone estrogen. It is primarily found in tissues such as the breast, uterus, and bone. When estrogen binds to ERα, it activates specific genes that regulate various biological processes, including reproduction, growth, and metabolism. ERα is significant in medical research, particularly in understanding conditions like breast cancer. Many breast cancer cells have high levels of ERα, making them responsive to estrogen. This knowledge has led to targeted therapies that block ERα activity, helping to treat hormone-sensitive breast cancers effectively.