Steroid and thyroid hormone receptors.

The identification, cloning, and characterization of steroid and thyroid hormone receptors have revealed much about the mechanisms of hormone action. While their remarkable similarities reflect evolutionary conservation of useful structural and functional motifs, differences in their gene number and organization, mRNA regulation, nuclear localization, target gene specificity, and transcriptional activation properties undoubtedly stem from the duplication and subsequent divergence that follows from the specialization of function in higher organisms. Further study of the nuclear receptors will clarify the shared and individual strategies used by hormones to regulate gene expression. It will be particularly important to understand the actual molecular mechanisms by which simultaneous binding of hormone and target gene by a receptor signals the transcriptional apparatus. Finally, the identification of novel putative receptors indicates that the biology of hormones that act in the cell nucleus will be even more complicated and exciting than ever imagined.