Nuclear receptors (Homo sapiens)

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Nuclear receptors are a class of proteins found within the interior of cells that are responsible for sensing the presence of steroid and thyroid hormones and certain other molecules. In response, these receptors work in concert with other proteins to regulate the expression of specific genes thereby controlling the development, homeostasis, and metabolism of the organism. Nuclear receptors have the ability to directly bind to DNA and regulate the expression of adjacent genes, hence these receptors are classified as transcription factors. The regulation of gene expression by nuclear receptors only happens when a ligand—a molecule which affects the receptor's behavior—is present. More specifically, ligand binding to a nuclear receptor results in a conformational change in the receptor which in turn activates the receptor resulting in up-regulation of gene expression. A unique property of nuclear receptors which differentiate them from other classes of receptors is their ability to directly interact with and control the expression of genomic DNA. Consequently nuclear receptors play key roles in both embryonic development and adult homeostasis. As discussed in more detail below, nuclear receptors may be classified either according to mechanism or homology. [source: Wikipedia]