The ERRs are orphan nuclear receptors, meaning the identity of their endogenous ligand has yet to be unambiguously determined. They are named because of sequence homology with estrogen receptors, but do not appear to bind estrogens or other tested steroid hormones.
ERRs bind to estrogen response elements and modulate transcription but are not activated by estrogens. They also have their own response elements: ERREs. ERRs have been associated with energy homeostasis, diabetes and cancer.