Fig. 2. Regression of the anterior boundaries of expression of construct CHZ. (A) By 8.5 dpc, anterior limits of expression were established at the hindbrain/spinal cord boundary in the neural tube, and at so6/7 in the paraxial mesoderm. The latter corresponds to the boundary of Hoxb4 expression. (B) Expression was noticeably weaker at the anterior boundaries by 8.75 dpc. (C-E) Boundaries continued to shift posteriorly until 9.5 dpc, coming to rest alongside so6/7 in the neural tube and at so13/14 in the paraxial mesoderm. Black arrowheads indicate the position of so7. ov, otic vesicle; fl, forelimb bud.