The T-box genes constitute an evolutionarily conserved family of putative transcription factors which are expressed in discrete domains during embryogenesis, suggesting that they may play roles in inductive interactions. Members have been identified by virtue of their homology to the prototypical T-box gene, T or Brachyury, which is required for mesoderm formation and axial elongation during embryogenesis. We have previously reported the discovery of six new mouse T-box genes, Tbx1-Tbx6, and described the expression patterns of Tbx1-Tbx5 (Bollag et al., 1994; Agulnik et al., 1996; Chapman et al., 1996; Gibson-Brown et al., 1996). We have obtained cDNA clones encoding the full-length Tbx6 protein from screens of gastrulation-stage mouse cDNA libraries and determined the spatial and temporal distribution of Tbx6 transcripts during embryogenesis. The gene codes for a 1.9-kb transcript with an open reading frame coding for a 540-amino acid protein, with a predicted molecular weight of 59 kDa. Tbx6 maps to chromosome 7 and does not appear to be linked to any known mutation. Unlike other members of the mouse T-box gene family which are expressed in a wide variety of tissues derived from all germ layers, Tbx6 expression is quite restricted. Tbx6 transcripts are first detected in the gastrulation stage embryo in the primitive streak and newly recruited paraxial mesoderm. Later in development, Tbx6 expression is restricted to presomitic, paraxial mesoderm and to the tail bud, which replaces the streak as the source of mesoderm. Expression in the tail bud persists until 12. 5 days postcoitus. Tbx6 expression thus overlaps that of Brachyury in the primitive streak and tail bud, although Brachyury is expressed earlier in the primitive streak. Brachyury is also expressed in a second domain, the node and notochord, that is not shared with Tbx6. The onset of Tbx6 expression is not affected in homozygous null Brachyury mutant embryos at 7.5 days postcoitus. However, Tbx6 expression is extinguished in mutant embryos as soon as the Brachyury phenotype becomes evident at 8.5 days postcoitus, indicating that the continued expression of Tbx6 is directly or indirectly dependent upon Brachyury expression.