Sex steroid binding proteins in non-mammalian vertebrates

High affinity cytosolic and nuclear steroid binding macromolecules have been identified in steroid target tissues of representative amphibians, reptiles and birds. In addition to these traditional "receptors" evidence exists which indicates the presence of other steroid binding proteins, varying in affinity and specificity, in the cytosol of steroid targets (liver, oviduct) of these same species. The possible relationships between cytosolic binding proteins of differing affinity and specificity and their relevance to hormone action in non-mammalian species is discussed.