ATF1 (Activating Transcription Factor 1, TREB-36) is a member of the ATF/CREB family of basic region leucine-zipper (bZip) DNA-binding proteins that regulates transcription by binding to a consensus cAMP response element (CRE) in the promoter of various viral and cellular genes. Many of these genes are important in cell growth and differentiation, and in stress and immune responses. The activation function of CRE-binding proteins may be modulated by phosphorylation of several kinases and is mediated by coactivators such as CREB-binding protein (CBP) and p300. ATF1 is a nuclear protein that binds DNA as a homodimer or as heterodimers with the inducible transcription factors CREB1 or CREM. Heterodimers appear to be stronger transcriptional activators than the homodimers. Tissue expression of ATF1 mRNA is widespread. Several isoforms of ATF1 arise by differential splicing. ATF1 mediates both Ca2+ and cAMP responses at several levels. It binds to the Tax-responsive element (TRE1) of the human T-cell lymphotropic virus type-I (HTLV1). ATF1 is detectable in metastatic melanoma cells and seems to contribute to their survival. A chimeric protein composed of the N-terminal domain of EWS (Ewing sarcoma oncogene) linked to the bZip domain of ATF1 is implicated in the rare malignant clear cell sarcoma of tendon sheath and aponeuroses (malignant melanoma of soft parts).