Three potential mechanisms to explain the time course of gene expression
	Our studies suggest that the thyroid hormone-gene interaction can be divided into three stages: Stage I, characterized by apparent lack of response to the hormone; Stage II, during which gene expression responds to T3; and Stage III, during which the gene is abundantly expressed but, curiously, not influenced by the presence or absence of T3. Whether these stages apply to all thyroid hormone responsive genes is not clear. We have also been intrigued by the mechanisms which determine the thyroid hormone response patterns to thyroid hormone observed. In particular, we were curious about the factors which allow the target genes to respond to hormone, i.e. 1) is the response of the gene determined by the level of thyroid hormone?, 2) is the response linked to the surge in TRb1 isoform?, or 3) is the competence of genes to respond to T3 related to other and as yet unidentified developmentally determined cellular factors?