Other estrogen metabolites like estriol, 17-epiestriol (produced in vivo) and genestein (a phytoestrogen) bind preferentially to ERβ [28]. 17-α-E2, a stereoisomer of 17-β-E2 binds to ERα and ERβ with forty fold lower affinity than 17-β-E2 in the neuronal cells [29]. Although both 17-α-E2 and 17-β-E2 bind to ER-X, it has been proposed that 17-α-E2 is the preferred endogenous ligand for ER-X in the neuronal cells [26].