Signal transduction networks allow cells to recognize and respond to changes in 
the extracellular environment. All eukaryotic cells have MAPK (mitogen-activated 
protein kinase) pathways that participate in diverse cellular functions, 
including differentiation, survival, transformation and movement. Five distinct 
groups of MAPKs have been characterized in mammals, the most extensively studied 
of which is the Ras/Raf/MEK [MAPK/ERK (extracellular-signal-regulated kinase) 
kinase]/ERK cascade. Numerous stimuli, including growth factors and phorbol 
esters, activate MEK/ERK signalling. How disparate extracellular signals are 
translated by MEK/ERK into different cellular functions remains obscure. 
Originally identified in yeast, scaffold proteins are now recognized to 
contribute to the specificity of MEK/ERK pathways in mammalian cells. These 
scaffolds include KSR (kinase suppressor of Ras), beta-arrestin, MEK partner-1, 
Sef and IQGAP1. Scaffolds organize multiprotein signalling complexes. This 
targets MEK/ERK to specific substrates and facilitates communication with other 
pathways, thereby mediating diverse functions. The adaptor proteins regulate the 
kinetics, amplitude and localization of MEK/ERK signalling, providing an 
efficient mechanism that enables an individual extracellular stimulus to promote 
a specific biological response.