The mouse cDNA for apoptosis signal-regulating kinase 1 (ASK)1 was isolated. The overall amino acid sequence identity between the mouse and the human ASK1 was 91.9%. A database search revealed that the kinase domain of ASK1 is evolutionally well-conserved over species among nematode, fly, mouse and human. Northern blot analysis identified a 6-kb transcript of ASK1 which is expressed in the various mouse adult tissues including heart, brain, lung, liver and kidney. Immunohistochemical analysis of mouse embryos (17 days post coitum) revealed a localized expression of ASK1 in developing skin, cartilage and bone, suggesting a possible role for ASK1 in tissue development during embryogenesis as well as cytokine-induced apoptosis.