The specification of the R7 photoreceptor cell in the developing eye of Drosophila is dependent upon activation of the Sevenless (SEV) receptor tyrosine kinase. By screening for mutations that suppress signaling via a constitutively activated SEV protein, we have identified a novel gene, daughter of sevenless (dos). DOS is required not only for signal transduction via SEV but also in other receptor tyrosine kinase signaling pathways throughout development. The presence of an amino-terminally located pleckstrin homology domain and many potential tyrosine phosphorylation sites suggests that DOS functions as an adaptor protein able to interact with multiple signaling molecules. Our genetic analysis demonstrates that DOS functions upstream of Ras1 and defines a signaling pathway that is independent of direct binding of the DRK SH2/SH3 adaptor protein to the SEV receptor tyrosine kinase.