Estrogen receptors (ERs) are proteins found in cells that function as receptors for the hormone estrogen (17β-estradiol). There are two main classes of ERs. The first includes the intracellular estrogen receptors, namely ERα and ERβ, which belong to the nuclear receptor family. The second class consists of membrane estrogen receptors (mERs), such as GPER (GPR30), ER-X, and G_q-mER, which are primarily G protein-coupled receptors. This article focuses on the nuclear estrogen receptors (ERα and ERβ).

Upon activation by estrogen, intracellular ERs undergo translocation to the nucleus where they bind to specific DNA sequences. As DNA-binding transcription factors, they regulate the activity of various genes. However, ERs also exhibit functions that are independent of their DNA-binding capacity. These non-genomic actions contribute to the diverse effects of estrogen signaling in cells.

Estrogen receptors (ERs) belong to the family of steroid hormone receptors, which are hormone receptors for sex steroids. Along with androgen receptors (ARs) and progesterone receptors (PRs), ERs play crucial roles in regulating sexual maturation and gestation. These receptors mediate the effects of their respective hormones, contributing to the development and maintenance of reproductive functions and secondary sexual characteristics.

In humans, the two forms of the estrogen receptor are encoded by different genes, ESR1 and ESR2 on the sixth and fourteenth chromosome (6q25.1 and 14q23.2), respectively.

There are two different forms of the estrogen receptor, usually referred to as α and β, each encoded by a separate gene (ESR1 and ESR2, respectively). Hormone-activated estrogen receptors form dimers, and, since the two forms are coexpressed in many cell types, the receptors may form ERα (αα) or ERβ (ββ) homodimers or ERαβ (αβ) heterodimers. Estrogen receptor alpha and beta show significant overall sequence homology, and both are composed of five domains designated A/B through F (listed from the N- to C-terminus; amino acid sequence numbers refer to human ER).^{[citation needed]}

The N-terminal A/B domain is able to transactivate gene transcription in the absence of bound ligand (e.g., the estrogen hormone). While this region is able to activate gene transcription without ligand, this activation is weak and more selective compared to the activation provided by the E domain. The C domain, also known as the DNA-binding domain, binds to estrogen response elements in DNA. The D domain is a hinge region that connects the C and E domains. The E domain contains the ligand binding cavity as well as binding sites for coactivator and corepressor proteins. The E-domain in the presence of bound ligand is able to activate gene transcription. The C-terminal F domain function is not entirely clear and is variable in length.^{[citation needed]}

Due to alternative RNA splicing, several ER isoforms are known to exist. At least three ERα and five ERβ isoforms have been identified. The ERβ isoforms receptor subtypes can transactivate transcription only when a heterodimer with the functional ERß1 receptor of 59 kDa is formed. The ERß3 receptor was detected at high levels in the testis. The two other ERα isoforms are 36 and 46kDa.

Only in fish, but not in humans, an ERγ receptor has been described.