The isolation of the mouse Brachyury (T) gene has provided an important molecular tool for the investigation of mesoderm formation and axial development in vertebrates. The T gene is expressed specifically in nascent and early migrating mesoendoderm, and in notochord cells. It acts as a transcription factor controlling the differentiation of notochord cells, and the formation of mesoderm in the posterior of the embryo. Thus, mouse embryos lacking T function cannot undergo trunk nor tail formation. T plays distinct roles in the two cell types.