We have isolated a dominant mutation in a gene called Son of sevenless (Sos) that is an allele-specific suppressor of the sevenless phenotype. This suppressor function is autonomously required in R7 and is sensitive to the dosage of the Sos and bride of sevenless genes. Loss-of-function alleles of Sos are recessive lethals, but in the eye Sos has a role in R cell development. Mutations in Sos also interact with the Ellipse allele of the Drosophila EGF receptor. We propose a model suggesting that the Sos product is downstream of sevenless and the EGF receptor, and that the dominant suppression results from the overexpression or increased activity of the gene product.