A 2007 study investigated the interaction between bisphenol A's and estrogen-related receptor γ (ERR-γ). This orphan receptor (endogenous ligand unknown) behaves as a constitutive activator of transcription. BPA seems to bind strongly to ERR-γ ( dissociation constant = nM), but only weakly to the ER.  BPA binding to ERR-γ preserves its basal constitutive activity.  It can also protect it from deactivation from the SERM 4-hydroxytamoxifen (afimoxifene).  This may be the mechanism by which BPA acts as a xenoestrogen .  Different expression of ERR-γ in different parts of the body may account for variations in bisphenol A effects. For instance, ERR-γ has been found in high concentration in the placenta , explaining reports of high bisphenol accumulation in this tissue.  BPA has also been found to act as an agonist of the GPER (GPR30).