Estradiol (E2), a key steroid hormone, regulates growth, differentiation, and function in various tissues, including the reproductive system, mammary gland, and cardiovascular system. It acts through two receptors, ERα and ERβ, which share some functional domains but differ in structure and expression. ERα is predominantly expressed in the breast, uterus, and cervix, while ERβ is found in the ovary, prostate, and brain. ER knockout mice show distinct phenotypes, indicating the unique roles of each receptor. ERα is essential for estrogen signaling in many tissues, while ERβ may have distinct functions, especially in the brain and reproductive system. The mechanisms of estrogen signaling include classical ligand-dependent pathways, ligand-independent pathways, DNA-binding-independent pathways, and cell-surface (nongenomic) signaling. Ligand-dependent signaling involves ER binding to DNA response elements (ERE) and activating gene transcription. Ligand-independent signaling occurs through interactions with growth factors and intracellular signaling pathways. DNA-binding-independent signaling involves ER interacting with other transcription factors, such as AP-1, to regulate gene expression. Nongenomic signaling involves ER on the cell surface, leading to rapid tissue responses. ERα and ERβ have distinct activation domains, with ERα containing AF-1 and AF-2 domains, while ERβ has a similar AF-2 domain but lacks AF-1. ERα is more active in estrogen signaling, but ERβ can compensate in some pathways. ERβ may also interact with EREs independently of ligand, potentially modulating ERα activity. Estrogen receptor modulators (SERMs) like tamoxifen and raloxifene have mixed agonist/antagonist activities, depending on the tissue and cellular context. SERMs can act as agonists in some tissues (e.g., bone, cardiovascular system) and antagonists in others (e.g., breast, uterus). Recent studies suggest that ERβ may also play a role in estrogen signaling, particularly in the cardiovascular and skeletal systems. The discovery of ERβ's role in estrogen signaling highlights the complexity of estrogen actions and the importance of understanding ER subtypes in developing targeted therapies. ER knockout models provide insights into the distinct functions of ERα and ERβ, and the development of SERMs aims to harness these differences for therapeutic benefit. Continued research into ER signaling mechanisms will enhance our understanding of estrogen's diverse biological effects and aid in the development of more effective treatments for estrogen-related disorders.Estradiol (E2), a key steroid hormone, regulates growth, differentiation, and function in various tissues, including the reproductive system, mammary gland, and cardiovascular system. It acts through two receptors, ERα and ERβ, which share some functional domains but differ in structure and expression. ERα is predominantly expressed in the breast, uterus, and cervix, while ERβ is found in the ovary, prostate, and brain. ER knockout mice show distinct phenotypes, indicating the unique roles of each receptor. ERα is essential for estrogen signaling in many tissues, while ERβ may have distinct functions, especially in the brain and reproductive system. The mechanisms of estrogen signaling include classical ligand-dependent pathways, ligand-independent pathways, DNA-binding-independent pathways, and cell-surface (nongenomic) signaling. Ligand-dependent signaling involves ER binding to DNA response elements (ERE) and activating gene transcription. Ligand-independent signaling occurs through interactions with growth factors and intracellular signaling pathways. DNA-binding-independent signaling involves ER interacting with other transcription factors, such as AP-1, to regulate gene expression. Nongenomic signaling involves ER on the cell surface, leading to rapid tissue responses. ERα and ERβ have distinct activation domains, with ERα containing AF-1 and AF-2 domains, while ERβ has a similar AF-2 domain but lacks AF-1. ERα is more active in estrogen signaling, but ERβ can compensate in some pathways. ERβ may also interact with EREs independently of ligand, potentially modulating ERα activity. Estrogen receptor modulators (SERMs) like tamoxifen and raloxifene have mixed agonist/antagonist activities, depending on the tissue and cellular context. SERMs can act as agonists in some tissues (e.g., bone, cardiovascular system) and antagonists in others (e.g., breast, uterus). Recent studies suggest that ERβ may also play a role in estrogen signaling, particularly in the cardiovascular and skeletal systems. The discovery of ERβ's role in estrogen signaling highlights the complexity of estrogen actions and the importance of understanding ER subtypes in developing targeted therapies. ER knockout models provide insights into the distinct functions of ERα and ERβ, and the development of SERMs aims to harness these differences for therapeutic benefit. Continued research into ER signaling mechanisms will enhance our understanding of estrogen's diverse biological effects and aid in the development of more effective treatments for estrogen-related disorders.