While scaffold proteins are thought to be key components of signaling pathways, 
their exact function is unknown. By preassembling multiple components of 
signaling cascades, scaffolds are predicted to influence the efficiency and/or 
specificity of signaling events. Here we analyze a potential scaffold of the 
Ras/mitogen-activated protein kinase (MAPK) pathway, kinase suppressor of Ras 
(KSR), by generating KSR-deficient mice. KSR-deficient mice were grossly normal 
even though ERK kinase activation was attenuated to a degree sufficient to block 
T-cell activation and inhibit tumor development. Consistent with its role as a 
scaffold, high-molecular-weight complexes containing KSR, MEK, and ERK were lost 
in the absence of KSR. This demonstrates that KSR is a bona fide scaffold that 
is not required for but enhances signaling via the Ras/MAPK signaling pathway.