The dm-Jun gene is the Drosophila homolog of the vertebrate c-jun gene. has been recently demonstrated that it participates in the decision between neuronal and non-neuronal cell fates in Drosophila eye development (Bohmann et al. 1994). Together with the transcription factors pet and yan, jun is required for normal photoreceptor determination in Drosophila, and all of them are regulated by the Ras/MAPK pathway. The aim of this project is to identify targets of the Jun transcription factor in the Ras signalling pathway. It has been shown that the expression of a constitutively active mutant Jun causes a dose-dependent development of extra photoreceptors (Treier et al., in preparation). Changes in this dominant phenotype obtained by mutagenesis will allow us to identify genes dowstream of Jun. Mutants will be generated by X-ray irradiation, the chemical mutagen ethylmethanesulfonate (ETS) and existing P-elements will be screened. They will be further analyzed at the structural and molecular level.