Multiple alternative interactions between activators and co-activators stimulate transcription by RNA polymerase II. In the past two years, multiprotein co-activator complexes have been characterized and their subunits defined. TATA-box binding protein associated factor (TAF) subunits of yeast TFIID were found to be generally required for transcription in vivo. Mammalian multisubunit coactivator complexes with homologs of the yeast SRB/Mediator subunits have been characterized. Structures of nuclear receptor-coactivator complexes have been determined.