Scheme to block BMP signaling conditionally. In the absence of 17β-estradiol (left), ER-xSmad6 is held in an inactive conformation via binding of Hsp90 (square) to the hER ligand-binding domain, allowing signaling (arrows) to occur normally. In the presence of hormone (small circle), ER-xSmad6 is released and blocks signaling downstream of the BMP receptors (right).