Two-component signal transducers, which are characterized by the histidine-to-aspartate phospho-transfer mechanism, were once thought to be restricted to prokaryotes. They have, however, now been identified in diverse eukaryotic species including plant, fungus, yeast and slime mold. In yeast, a two-component osmosensor has been found to regulate a mitogen-activated protein kinase (MAPK) cascade, a ubiquitous eukaryotic signaling module.