Endoxifen induces ERα/β heterodimer formation. MCF7-ERβ (A) or U2OS-ERα/β (B) cells were treated with indicated concentrations of endoxifen or vehicle for 24 hours. Equal amounts of cell lysates were immunoprecipitated with an ERβ specific antibody. Immunoprecipitated protein (IP) complexes were separated by SDS-PAGE and Western blotting (WB) was performed using an ERα specific antibody. Non-immunoprecipitated ERα and ERβ protein levels were also determined by Western blotting in whole cell extracts (WCE) following endoxifen treatment. Tubulin levels are shown as protein loading controls.