Hormonally induced increases in cyclic AMP levels induce phosphorylation of the transcription factor CREB at a serine residue at position 133 by protein kinase A (ref. 1), enhancing its ability to activate transcription without affecting its intracellular location or DNA-binding activity. This effect is dependent on a 60-amino-acid region of CREB that contains Ser133 and is termed the kinase-inducible domain (KID)2, which also occurs in the CREB-related CREM-alpha and -beta proteins, although these are transcriptional repressors. Here we show that the KID domain confers a cAMP-inducible increase on the activity of the Q2 activation domain from CREB and the acidic activation domains from the yeast proteins GAL4 and GCN4. Remarkably, it retains this ability even when attached to a separate polypeptide bound to an adjacent site in the promoter. KID may therefore be the first of a new class of conditional activators that work through other promoter-bound factors to stimulate gene expression in response to hormonal stimuli.