Activation of the Mek/Erk pathway in B-Raf+/LSLV600E mice. (A) Protein extracts obtained from B-Raf+/+ and B-Raf+/LSLV600E adult brain and heart tissues as well as E13.5 embryos were submitted to Western blot analyses using antibodies to B-Raf, pMek1/2, Mek1, pErk1/2, and Erk1/2. GAPDH was used as loading control. (B) Protein extracts obtained from B-Raf+/+ and B-Raf+/LSLV600E adult brain and heart tissues were incubated with a monoclonal antibody against B-Raf, and the resulting immunoprecipitates were assayed for kinase activity using Mek1 as a substrate. The levels of pMek1 were determined by blotting with specific polyclonal antibodies against pMek1. The same membrane was used for Western blot analyses using B-Raf antibody as loading control. Arrowheads indicate the migration of the corresponding proteins.