Nuclear hormone receptors regulate gene transcription upon recognizing specific regulatory sequences – hormone responsive elements (HRE) in gene promoters, enhancers, and silencers. Receptors for sexual and adrenal steroid hormones, thyroid hormone, retinoic acid and vitamine D, as well as an extensive group of orphan receptors, all exhibit strong homology in structural and functional organization of the molecule and they form together the nuclear receptor superfamily. While classical steroid receptors dissociate upon activation an inhibitory hsp-complex, dimerize and then are able to bind cognate HRE and activate transcription, thyroid and retinoid receptors as well as the vitamine D receptor bind the HRE in question constitutively and activation is represented by dissociating a corepressor and recruiting a coactivator. Similar mode of action applies also for orphan receptors. Some of recently resolved orphan receptors are activated by immediate products of metabolism; the metabolism of cholesterol relies in great part on selective activation of a particular orphan receptors by a particular cholesterol metabolite.

        Key words: transcription regulation, nucleare hormone receptors, signal transduction, orphane receptors