Embryos were injected with ER-xSmad6 mRNA at the 4-cell stage into either the 2 presumptive dorsal or ventral blastomeres (n = number of embryos) and either cultured alone or induced at stage 10.5 with E2. Some leakiness of the fusion protein is evident in the ventral-targeted injections cultured in the absence of E2, but there is a clear trend toward blocking of either anterior or posterior domains by activated ER-xSmad6.