Activating transcription factor 3 is a member of the mammalian activation transcription factor/cAMP responsive element-binding (CREB) protein family of transcription factors. It encodes a protein with a calculated molecular mass of 22 kD. ATF3 represses rather than activates transcription from promoters with ATF binding elements. An alternatively spliced form of ATF3 (ATF3 delta Zip) encodes a truncated form ATF3 protein lacking the leucine zipper protein-dimerization motif and does not bind to DNA. In contrast to ATF3, ATF3 delta Zip stimulates transcription presumably by sequestering inhibitory co-factors away from the promoter. It is possible that alternative splicing of the ATF3 gene may be physiologically important in the regulation of target genes. Three transcript variants encoding two different isoforms have been found for this gene.