Ethan Bier of the embryo, but as a repressor of genes expressed in Department of Biology dorsal cells. In ventral cells, the mesoderm-determining and Center for Molecular Genetics genes twist and snail are activated by peak levels of University of California, San Diego Dorsal. Lower levels of Dorsal activate expression of 9500 Gilman Drive rhomboid (rho) in lateral presumptive neuroectodermal L...