TR isoform selectivity for TRE sequences in genes thar mediate thyroid hormone response have been seen in some studies, but not all. TRE sequences influence TR isoform interaction with ligand (36) and may influence coactivator recruitment (37), TR interaction with TREs is not static; as has been reported with other nuclear receptors, there is variation in the pattern of binding that may be influenced by the TRE (37).In vitro studies have shown some TR isoform preferences for specific TREs (38), although the ability to translate these findings to in vivo observations are likely limited. Liver gene perofiling in TRcc and TR gene knockouts demonstrates little in the way of spicific genes linked to a TR isoform (25). A recent study, however, suggests that the relative potency of activation may be controlled more by the relative expression of TR or TR in a tissue, rather than by TR isoform specificity for a specific TRE (39).