Molecular genetic studies of C. elegans vulval development have helped to define an evolutionarily conserved signaling pathway from an EGF-like ligand through EGF-receptor, Ras and MAP kinase to the nucleus. Further studies have identified novel positive regulators such as KSR-1 and SUR-8/SOC-2 and negative regulators such as cbl/SLI-1. The many negative regulatory proteins might serve to prevent inappropriate signaling, and thus are analogous to tumor suppressor genes.