AP-1 family transcription factor that controls the differentiation of lineage-specific cells in the immune system: specifically mediates the differentiation of T-helper 17 cells (Th17), follicular T-helper cells (TfH), CD8(+) dendritic cells and class-switch recombination (CSR) in B-cells. Present in the nucleus and cytoplasm, but shows increased nuclear translocation after activation of T-cells. Belongs to the bZIP family.