Recent studies indicate that kinase suppressor of Ras (KSR)is a scaffold protein 
for the Ras/Raf/MEK/ERK signaling cascade in mammals. To help determine the in 
vivo function of KSR, we have examined the tissue-specific distribution of this 
protein in the embryonic and adult mouse using a rat monoclonal antibody raised 
against the mouse protein. Western blot analysis indicates that the protein is 
expressed at highest levels in the adult brain. It is also expressed at low 
levels in bladder, ovary, testis, and lung, but the protein is not detectable in 
any other adult tissue. However, reverse transcription-PCR analysis shows that 
Ksr transcripts are detected in all adult tissues except the liver. A variant 
containing a differentially spliced exon in the CA4 domain is observed in brain, 
cerebellum, ovary, and intestine. The protein is also expressed throughout the 
E6.5 embryo and at high levels in the neuroepithelium of the E10.5 embryo. At 
this embryonic stage, expression is also detected at lower levels in the limb 
and tail buds as well as in the myocardium.