The aim of this project is to elucidate some of the molecular mechanisms that underlie the early definition of dorso-ventral axis in vertebrates, using Xenopus as a model system. The Nieuwkoop Center (NC), located in the dorsal-vegetal cells of blastulae, constitutes the first organizing center during embryogenesis and it is defined by the relocation of maternal of dorsal determinants. It acts by inducing Spemann's organizer in the overlying mesodermal cells. In spite of its central role in dorso-ventral axis formation, the mechanisms involved in the definition of the NC are unclear. This project proposes to tackle this issue by studying the spatial regulation of Siamois, a homebox gene expressed in the NC. After cloning the genomic sequences of Siamois, we will define the cis regulatory elements present that are important for its spatial expression. Once determined, it will be possible to purify the maternal determinants that bind these sequences.