Siamois and Twin are redundant and essential in formation of the Spemann organizer

►Goosecoid transcription is directly activated by Siamois and Twin. ►Siamois and Twin bind a conserved P3 site of the endogenous Goosecoid promoter. ►Siamois-Twin homodimers and heterodimers have equivalent function. ►Siamois and Twin are redundant and essential for Spemann organizer formation. ►The transcriptional response to maternal Wnt signals is mediated by Siamois and Twin.