Two other recently studied human NRs appear to bind to their ligands in the micromolar
range of equilibrium dissociation constants, suggesting the possibility that they also function
as physiological sensors for signaling molecules. In the case of the Rev-erb-α and -β
receptors, iron porphyrin IX has been shown by three groups to act as the endogenous
ligands, regulating the corepressor binding and gene repression activities of these receptors
(9–11).