Brachyury is a transcription factor encoded by T, a member of the T-box gene family, and required for posterior mesoderm formation and differentiation as well as for notochord development. It is expressed in all nascent mesoderm, in the embryo. Along with embryonic differentiation, it is downregulated, restricted into the notochord and the tail bud, and then lost, when notochordal cells are replaced by bone in the vertebral bodies and by the nucleus pulposus in the intervertebral discs. In the adult, brachyury expression has been found in chordoma, a malignant tumor which recapitulates notochord and derives from small collections of notochordal cells which may persist into the adult life.