T-box transcription factor that plays an essential role in the determination of the fate of axial stem cells: neural vs mesodermal. Acts in part by down-regulating, a specific enhancer (N1) of SOX2, to inhibit neural development. Seems to play also an essential role in left/right axis determination and acts through effects on Notch signaling around the node as well as through an effect on the morphology and motility of the nodal cilia. TBX6 is first detected in the gastrulation stage in the primitive streak and newly recruited paraxial mesoderm. Later in development it is restricted to presomitic, paraxial mesoderm and to the tail bud, which replaces the streak as the source of mesoderm. Defects in the differentiation of paraxial mesoderm. Irregular somites formed in the neck region of mutant embryos, whereas more posterior paraxial tissue did not form somites but instead differentiated along a neural pathway, forming neural-tube-like structures that flanked the axial neural tube. These paraxial tubes showed dorsal/ventral patterning that is characteristic of the neural tube and had differentiated motor neurons. Embryos lacking TBX6 show also randomization of the direction of heart looping and independent alterations in the direction of embryo turning.