Figure 2.

NR3 class of nuclear receptors. NR3 class of nuclear receptors and their corresponding ligands are shown. NR3 class of nuclear receptors are known as ‘nuclear steroid receptor’ because all the physiological ligands for them have a steroid backbone in their structural formula. Estrogen-related receptors (ERRs), for which physiological ligands do not exist or have not been identified, are referred to as orphan receptors. ER (estrogen receptor) subtypes (ERα and ERβ) and ERR subtypes (ERRα, ERRβ, and ERRγ) are coded in different genes. GR (glucocorticoid receptor) isoforms (GRα and GRβ) are splicing variants coded by the same gene. PR (progesterone receptor) isoforms (PR-A, PR-B, and PR-M) are derived from the same gene with distinctive transcription initiation sites. PR-M (mitochondrial progesterone receptor) contains a unique N-terminal amino acid sequence suggestive of a transmembrane domain (green). NTD; N-terminal domain (red), DBD; DNA-binding domain (orange), H; hinge region (purple), LBD; ligand-binding domain (blue), a.a.; amino acids.