ER-xSmad6 specifically blocks BMP signaling. (A) The luciferase reporter constructs p3TP-Lux or pBRE-Luc were injected alone or coinjected into 2-cell stage embryos with mRNA encoding BMP4, activin βB, ΔBR, or xSmad6 (“Materials and methods”). a indicates p3TP-Lux alone is significantly different from p3TP-Lux + activin (P < .001). (B-C) ER-xSmad6 (1 ng) was coinjected at the 2-cell stage with either p3TP-Lux and activin βB (B) or pBRE-Luc and xBMP4 (C) and cultured in the presence or absence of E2 from the time of injection. In panel B, a indicates p3TP-Lux alone is significantly different from p3TP-Lux + activin (P < .001) and ** indicates the values are not significant compared with p3TP-Lux + activin. In panel C, ** indicates the value is not significant compared with pBRE-Luc + xBMP4, whereas * indicates the value is significant (P < .001). (A-C) All values reported as luciferase value normalized to control Renilla value. Error bars represent one standard deviation above the mean.