Estrogen receptor-related receptors (ERRs), including ERRα, ERRβ, and ERRγ, belong to the subfamily of orphan NR and share sequence homology with ERs. However, ERRs do not respond to ligands of ER such as estradiol . ERRα is the oldest orphan NR that has long been thought to not require an endogenous ligand, because it is active as long as the cells or tissues express its coactivators such as PGC-1α/β.