T-box transcription factor T, also known as Brachyury protein, is encoded for in humans by the TBXT gene. Brachyury functions as a transcription factor within the T-box family of genes. The gene brachyury appears to have a conserved role in defining the midline of a bilaterian organism, and thus the establishment of the anterior-posterior axis; this function is apparent in chordates and molluscs. Its ancestral role, or at least the role it plays in the Cnidaria, appears to be in defining the blastopore. It also defines the mesoderm during gastrulation. Tissue-culture based techniques have demonstrated one of its roles may be in controlling the velocity of cells as they leave the primitive streak. It effects transcription of genes required for mesoderm formation and cellular differentiation.[clarification needed] Brachyury has also been shown to help establish the cervical vertebral blueprint during fetal development. The number of cervical vertebrae is highly conserved among all mammals; however a spontaneous vertebral and spinal dysplasia (VSD) mutation in this gene has been associated with the development of six or fewer cervical vertebrae instead of the usual seven.