Developmentally regulated transcription factor AP-1 binds and recognizes the enhancer DNA sequence: 5'-TGA[CG]TCA-3'. May play a role in the function or determination of a particular subset of cells in the developing embryo. It is able to carry out its function either independently of or in conjunction with Jra. Homodimer. Heterodimer with Jra. The kay-Jra heterodimer binds more stably to the AP-1 site than either of the two proteins alone. Early expression in the embryo is mesodermal and some of this expression is localized to a region surrounding the cephalic furrow. Later in embryonic development expression is ectodermal, corresponding to muscle attachment sites. Also observed in part of the mid- and hindgut and in the anal pad. Isoform A and isoform B are expressed both maternally and zygotically. Zygotically expressed throughout embryogenesis, until second larval instar. Isoform A is more highly expressed than isoform B. Loss of isoform A causes embryonic lethality. Mammals typically have four copies of fos, Drosophila has a single gene with multiple transcription start sites giving rise to multiple protein isoforms. Produced by alternative promoter usage. Produced by alternative promoter usage. Belongs to the bZIP family. Fos subfamily.