Brachyury is the founder member of the T-box family of transcription factors, which is characterized by a DNA-binding domain of approximately 200 amino acids. Members of the T-box gene family play important roles in the development of both vertebrate and invertebrate embryos, including the control of gastrulation, development of the heart, and perhaps even the decision as to whether to form arm or leg. An understanding of how the T-box genes act requires analysis of how their expression is controlled, identification of their target genes, and an insight into how different family members exert different effects.