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. Isoform D and isoform sro are expressed both maternally and zygotically. Zygotic expression is present throughout embryogenesis, fading by second larval instar. Isoform D has higher expression level than isoform sro. Loss of isoform D 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. May perform a regulatory function, acting as a dominant negative regulator of isoform alpha. Belongs to the bZIP family. Fos subfamily.