Wnt3a encodes a signal that is expressed in the primitive streak of the gastrulating mouse embryo and is required for paraxial mesoderm development. In its absence cells adopt ectopic neural fates. Embryos lacking the T-boxcontaining transcription factors, Brachyury or Tbx6, also lack paraxial mesoderm. Here we show that Brachyury is specifically down-regulated in Wnt3a mutants in cells fated to form paraxial mesoderm. Transgenic analysis of the T promoter identifies T (Brachyury) as a direct transcriptional target of the Wnt signaling pathway. Our results suggest that Wnt3a, signaling via Brachyury, modulates a balance between mesodermal and neural cell fates during gastrulation. Received September 13, 1999; revised version accepted October 13, 1999. The embryonic mesoderm of the mammalian embryo is formed by a series of inductive interactions first in the primitive streak, which gives rise to head and trunk mesoderm, and later in the tailbud, which generates the most posterior mesoderm of the tail. As development progresses, successively posterior structures are generated, leading to a posterior extension of the body axis. Previous studies have established that Wnt3a, which encodes a member of the Wnt family of secreted signaling molecules (for review, see Cadigan and Nusse 1997;Moon et al. 1997), is expressed in pluripotent ectoderm cells of the primitive streak during gastrulation (Takada et al. 1994). At early somite stages [8.0-8.5 days postcoitum (dpc)], the Wnt3a expression domain correlates with a domain of cells in the anterior primitive streak fated to give rise to paraxial mesoderm (for review, see Tam and Trainor 1994;Wilson and Beddington 1996). Moreover, the anterior and lateral limits of the Wnt3a expression domain lie between cells fated to give rise to paraxial mesoderm and cells that will give rise to neural ectoderm.A requirement for Wnt3a in the specification of trunk and tail paraxial somitic mesoderm fates has been demonstrated by mutant analyses. Wnt3a homozygous null mutant embryos lack all but the anterior-most seven to nine somites (Takada et al. 1994;Greco et al. 1996;Yoshikawa et al. 1997). As a consequence, only the most rostral cervical vertebrae are formed. Histological and molecular analyses demonstrate that ectopic neural structures form in place of posterior paraxial mesoderm (Yoshikawa et al. 1997 Mesoderm specification is thought to be regulated, at least in part, by members of the T-box gene family of DNA-binding transcription factors (Smith 1999). Two of these, T and Tbx6, are coexpressed with Wnt3a in the primitive streak during gastrulation (Takada et al. 1994;Chapman et al. 1996). Mutations in either gene lead to a loss of trunk and tail mesoderm (Chesley 1935;Chapman and Papaioannou 1998). Ectopic neural tubes form in place of paraxial mesoderm in the Tbx6 mutants, but it is not clear how similar the neural tube abnormalities noted in the T homozygotes are to the Wnt3a phenotype. Given the similarities between the Wnt3a and T-box mutant phenotypes, we have investiga...