Estrogens can potentially be classified into planar (Class I) or non planar (Class II) categories, which might have biological consequences. 1,1,2-Triphenoplsylethylene (TPE) derivatives were synthesized and evaluated against 17β-estradiol (E2) for their estrogenic activity in MCF-7 human breast cancer cells. All TPEs were estrogenic and unlike 4-hydroxytamoxifen (4OHTAM) and endoxifen, induced cell growth to a level comparable to that of E2. All the TPEs increased ERE activity in MCF-7:WS8 cells with the order of potency as followed: E2 > 1,1-Bis(4,4′-hydroxyphenyl)-2-phenylbut-1-ene (15) > 1,1,2-Tris(4-hydroxyphenyl)but-1-ene (3) > Z 4-(1-(4-hydroxyphenyl)-1-phenylbut-1-en-2-yl)phenol (7) > E 4-(1-(4-hydroxyphenyl)-1-phenylbut-1-en-2-yl)phenol (6) > Z(4-(1-(4-ethoxyphenyl)-1-(4-hydroxyphenyl)but-1-en-2-yl)phenol (12) > 4-OHTAM. Transient transfection of the ER-negative breast cancer cell line T47D:C4:2 with wildtype ER or D351G ER mutant revealed that all of the TPEs increased ERE activity in the cells expressing the wild-type ER but not the mutant, thus confirming the importance of Asp351 for ER activation by the TPEs. The findings confirm E2 as a Class I estrogen and the TPEs as Class II estrogens. Using available conformations of the ER liganded with 4OHTAM or diethylstilbestrol, the TPEs optimally occupy the 4OHTAM ER conformation that expresses Asp351.