Transcriptional activator which regulates the transcription of various genes, including those involved in anti-apoptosis, cell growth, and DNA damage response. Dependent on its binding partner, binds to CRE (cAMP response element) consensus sequences (5'-TGACGTCA-3') or to AP-1 (activator protein 1) consensus sequences (5'-TGACTCA-3') (By similarity). Binds DNA as a dimer and can form a homodimer in the absence of DNA. Can form a heterodimer with JUN. Heterodimerization is essential for its transcriptional activity (By similarity). Belongs to the bZIP family. ATF subfamily.