The CREM gene (cAMP‐responsive element modulator) generates both activators and repressors of cAMP‐induced transcription by alternative splicing. We determined the exon structure of the CREM gene and have identified new isoforms. We show that CREM isoforms with different structural characteristics are generated by the shuffling of exons to produce proteins with various combinations of functional domains. CREM proteins bind efficiently to CREs and here we demonstrate that the various isoforms heterodimerize in vivo with each other and with CREB. The two alternative DNA binding domains of CREM, which are differentially spliced in the various isoforms, show distinct binding efficiencies, while CREM alpha/CREB heterodimers exhibit stronger binding than CREM beta/CREB heterodimers to a consensus CRE in vitro. We identify the protein domains involved in activation function and find that the phosphorylation domain and a single glutamine‐rich domain are sufficient for activation. A minimal CREM repressor, containing only the b‐Zip motif, efficiently antagonizes cAMP‐induced transcription. In addition, phosphorylation may reduce repressor function, as a CREM beta mutant carrying a mutation of the serine phosphoacceptor site (CREM beta 68) represses more efficiently than the wild‐type CREM beta.