Estrogen receptor beta (ERβ) also known as NR3A2 (nuclear receptor subfamily 3, group A, member 2) is one of two main types of estrogen receptor—a nuclear receptor which is activated by the sex hormone estrogen. ERβ may inhibit cell proliferation and opposes the actions of ERα in reproductive tissue. ERβ may also have an important role in adaptive function of the lung during pregnancy. ERβ is a potent tumor suppressor and plays a crucial role in many cancer types such as prostate cancer and ovarian cancer. ERβ function is related to various cardiovascular targets including ATP-binding cassette transporter A1 (ABCA1) and apolipoprotein A1 (ApoA-1). Polymorphism may affect ERβ function and lead to altered responses in postmenopausal women receiving hormone replacement therapy.