AP-1 family transcription factor that controls the differentiation of lineage-specific cells in the immune system. Following infection, participates in the differentiation of CD8(+) thymic conventional dendritic cells in the immune system. Acts via the formation of a heterodimer with JUN family proteins that recognizes and binds DNA sequence 5'-TGA[CG]TCA-3' and regulates expression of target genes (By similarity). Selectively suppresses CCN1 transcription and hence blocks the downstream cell proliferation signals produced by CCN1 and inhibits CCN1-induced anchorage-independent growth and invasion in several cancer types, such as breast cancer, malignant glioma and metastatic melanoma. Possibly acts by interfering with AP-1 binding to CCN1 promoter. Heterodimer; heterodimerizes with JUN family proteins. By type I interferon. Down-regulated in most hepatocellular carcinoma tumorous tissues (at protein level). Belongs to the bZIP family.