ATF3 (activating transcription factor 3) is a member of the mammalian activation transcription factor/cAMP responsive element-binding (CREB) protein family of transcription factors. This protein binds the cAMP response element (CRE) (consensus: 5'-GTGACGT[AC][AG]-3'), a sequence present in many viral and cellular promoters. Multiple transcript variants encoding two different isoforms have been found for this gene. The longer isoform represses rather than activates transcription from promoters with ATF binding elements. The shorter isoform (deltaZip2) lacks the leucine zipper protein-dimerization motif and does not bind to DNA, and it stimulates transcription, it is presumed, by sequestering inhibitory co-factors away from the promoter.