| Interestingly , CAPER exhibited a cryptic autonomous transactivation function that becomes activated only in the presence of estradiol bound ER . ^^^ This protein CAPER ( for coactivator of activating protein 1 ( AP 1 ) and estrogen receptors ( ERs ) ) selectively bound , among the many transcription factors we tested , the AP 1 component c Jun and the estradiol bound ligand binding domains of ERalpha and ERbeta . ^^^ In cotransfections , CAPER stimulated transactivation by ERalpha , ERbeta , and AP 1 . ^^^ |