MAPK modules are composed of a cascade of three intracellular protein kinases (MKKK, MKK and MAPK) which are activated successively by phosphorylation events. They are used to transduce a variety of information in organisms as diverse as yeasts, worms, flies or mammals. MAPK modules integrate signals coming from membrane receptor activation and, by the ability of MAPK to translocate into the nucleus and phosphorylate nuclear targets such as transcription factors, they relay extracellular signals into a genomic response. Since several MAPK modules transducing different information are expressed in the same cell, in yeast or in mammals, the question arises as to how fidelity is maintained between the distinct MAPK modules of a single cell. Two levels of specificity have been documented: the molecular selectivity of each enzyme for its substrate, which is particularly evident for the MKK-MAPK couple, permits specificity within one particular module; exogenous proteins, such as the yeast Ste5 protein, may serve as 'chaperone' proteins to tether all the members of a module and restrict signal transduction to this module. In mammalian cells, the MAPK modules are not strictly independent and one pathway may interfere with another. It remains to be determined whether this interference is of physiological relevance.