<unk>  (also known as<unk> -E2, 17-epiestradiol, alfatradiol, or estra-1,3,5(10)-triene-3,17α-diol) is a minor and weak endogenous steroidal estrogen that is related to<unk>  (better known simply as<unk> . It is the C17 epimer of<unk> . It has approximately 100-fold lower estrogenic potency than<unk> . The compound shows preferential affinity for the ERα over the ERβ. Although<unk>  is far weaker than<unk>  as an agonist of the nuclear estrogen receptors, it has been found to bind to and activate the brain-expressed ER-X with a greater potency than that of<unk> , suggesting that it may be the predominant endogenous ligand for the receptor.
17α-Estradiol
16α-Hydroxyestrone
11-Ketoprogesterone
11α-Hydroxyprogesterone
Alfatradiol
11β-Hydroxyprogesterone
Estradiol 
8,9-Dehydroestrone