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Role of FoxA4 and FoxA2 in dorsal axis development in Xenopus laevis Sabrina Murgan, Cecilia E. Aguirre, Silvia L. López, and Andrés E. Carrasco Laboratorio de Embriología Molecular, Instituto de Biología Celular y Neurociencias, Facultad de Medicina, Universidad de Buenos Aires. Paraguay 2155 Piso 3 (1121), Buenos Aires, Argentina The embryonic Dorsal Midline (DML), composed by the floor plate, the notochord and the hypochord, plays important roles in the patterning of dorsal tissues. Members of the FoxA subfamily of transcription factors are expressed in these structures. Foxa2 is essential for DML development in mammals, since knock-out mouse embryos lack a definitive node, notochord and floor plate(Ang and Rossant, 1994; Cell. 76, 561-574) (Weinstein et al., 1994; Cell 76, 575-566). Since the early expression pattern of Xenopus FoxA4 resembles more faithfully the expression of mouse Foxa2, it was suggested that the early functions of mouse Foxa2 are carried out by FoxA4 in frogs (Ruiz i Altaba et al., 1993; Mech. Dev. 44, 91-108). However, little is known about the function exerted by FoxA4 and FoxA2 in DML development in amphibians. The main goal of this work was to shed light on the role that FoxA4 and FoxA2 play in Xenopus DML development. We used in situ hybridization and knockdown by specific antisense morpholino oligonucleotides (MO). Gain of function studies were done for FoxA4. FoxA4MO injection, but not Foxa2MO, disrupted notochord and floor plate integrity and affected convergent extension. In the neuroectoderm, depletion of FoxA4 expanded the rostral forebrain, while its overexpression produced the opposite phenotype. We conclude that FoxA4 is necessary for the specification of the notochord and the floor plate and is required for central nervous system patterning in Xenopus.