D-raf, a Drosophila homolog of Raf-1, plays key roles in multiple signal transduction pathways. Dsor1, a putative factor downstream of D-raf, was genetically identified by screening of dominant suppressors of D-raf. Dsor1Su1 mapped on X chromosome significantly suppressed the D-raf mutant phenotypes, and the loss-of-function mutations of Dsor1 showed phenotypes similar to those of the D-raf null mutations. Dsor1Su1 also significantly suppressed the mutations of other terminal class genes acting further upstream of D-raf. Molecular cloning of Dsor1 revealed its product with striking similarity to the microtubule-associated protein (MAP) kinase activator and yeast PBS2, STE7, and byr1. Our genetic results demonstrate the connection between raf and the highly conserved protein kinase cascade involving MAP kinase in vivo.