Involved in specifying the paraxial, but not dorsal, mesoderm. May regulate the expression of T-box transcription factors required for mesoderm formation and differentiation. Specifically expressed in unsegmented paraxial mesoderm and its immediate progenitors and sharply down-regulated in presumptive somites. Not detectable at 6.5 dpc. At 7.5 dpc, expressed mainly in the posterior region of the embryo, lateral to the primitive streak where presumptive progenitors of paraxial mesoderm reside. Expression is largely excluded from the midline of the primitive streak. No expression is detected in the anterior part of the embryo. At 9.0 dpc, highly expressed in the caudal presomitic mesoderm. At 11.5 dpc, the primitive streak completely regresses, expression is observed exclusively in the tailbud. Expression remains in the tailbud until 13.5 dpc and begins to disappear between 13.5 and 14.5 dpc when the tailbud loses its potential to provide paraxial mesoderm cells. Mutant animals die during gestation. Premature death starts around 10.5 dpc. Severe morphological abnormalities, such as enlarged tailbud, kinked neural tube posterior to forelimb buds and reduced tissue mass in the interlimb domain become obvious at 9.0 dpc and thereafter. Head and anterior body structures, including the developing heart, appeared normal. In the trunk posterior to the forelimbs, there are no identifiable somites or segment patterning, although the anterior-most somites appeared normal, with decreases in somite size and disruption of myotomal patterning in somites 8-11. There are no detectable somites beyond somite 11. At 11.5 dpc, myotomes are absent from the trunk at the interlimb level and at 12.5 dpc, muscle is absent from the hindlimbs. The few fetuses surviving beyond 14.5 and 17.5 pdc lack a tail and show a grossly normal, but very thin body with relatively normal forelimbs and hindlimbs. At this stage, all vertebrae posterior to cervical level together with ribs are absent, while all cervical vertebrae are present, although the most posterior ones are sometimes malformed and fused. Other trunk skeletal structures that originate from lateral mesoderm, such as the sternum and scapula, form quite normally.