Steroid hormone receptor homologs in development.

The steroid/thyroid receptor superfamily are ligand-dependent transcription factors which consist of distinct functional domains required for transcriptional control of a network of genes. Members of this superfamily are beginning to be studied for their contribution to embryogenesis. Two human receptors for the vertebrate morphogen retinoic acid have been isolated and further characterized on model promoters. Moreover, the presence of homologs of these receptors in Drosophila reveals that members of this superfamily predate the divergence of the vertebrates and invertebrates. One locus is knirps-related (knrl), whose product is closely related to that of the gap segmentation gene knirps (kni). knrl is one of the most diverged steroid receptor-like molecules and displays a spatially restricted blastoderm pattern.