ABSTRACT
In1987,aretinoicacidreceptor(RARa.)thatbelongstothefamilyofsteroid/ thyroidhormonereceptorswasclonedindependentlybythegroupsofChambon andEvans(62,63).Thisfamilyofnuclearreceptorsconsistsofligand-dependent transcriptionfactorsthatregulategeneexpressionbybindingtoshortDNA sequences(hormone-responsiveelementsorenhancers)inthevicinitiesof targetgenes(64)(Figure9).SubsequentlywasahepatitisBvirusintegrationsite inahumanhepatocellularcarcinomashowntobeadifferentretinoicacidreceptor (RARI)).Finally,athirdretinoicacidreceptor,RARy,wascloned.Comparisonof theaminoacidsequencesofthethreereceptorsshowsthattheinterspecies conservation ofamemberoftheRARsubfamilyismuchhigherthantheconservationofaUthreereceptorswithinagivenspecies,suggestingthatRARa,RARI), andRARyeachhavetheirownspecificfunction(Chapters7and8).
