Signal transduction pathways constructed around a core module of three consecutive protein kinases, the most distal being a member of the extracellular signal-regulated kinase (ERK) family, are ubiquitous among eukaryotes. Recent work has defined two cascades activated preferentially by the inflammatory cytokines TNF-alpha and IL-1-beta, as well as by a wide variety of cellular stresses such as UV and ionizing radiation, hyperosmolarity, heat stress, oxidative stress, etc. One pathway converges on the ERK subfamily known as the "stress activated' protein kinases (SAPKs, also termed Jun N-terminal kinases, JNKs), whereas the second pathway recruits the p38 kinases. Upstream inputs are diverse, and include small GTPases (primarily Rac and Cdc42; secondarily Ras) acting through mammalian homologs of the yeast Ste20 kinase, other kinase subfamilies (e.g. GC kinase) and ceramide, a putative second messenger for certain TNF-alpha actions. These two cascades signal cell cycle delay, cellular repair or apoptosis in most cells, as well as activation of immune and reticuloendothelial cells.