Fos and Jun proteins homo- or heterodimerize to form functional AP-1 transcription factors. Drosophila mutants lacking either Jun or Fos display indistinguishable dorsal open phenotypes, indicating an essential function of both Jun and Fos for embryonic dorsal closure. Here we present experiments to determine the basis for this dual requirement. By combining mutant alleles and transgenes expressing Fos and Jun variants with altered dimerization preferences, fly lines were generated in which only specifically defined dimer variants can form. Phenotypic analysis of these mutants reveals that homodimers of Fos or of Jun cannot replace the function of the heterodimeric complex. This defect is not explained by the lower stability of homodimers as compared to heterodimers, because 'pseudo-homodimers' which are as stable as native Jun-Fos heterodimers cannot substitute for their function. We conclude that Jun and Fos play complementary roles that are both required for signal transduction and gene activation during dorsal closure.