B-Raf is required for ERK activation during postnatal brain development. (A) Expression/phosphorylation of components and targets of the ERK pathway in b-raf ^f/f (WT) and b-raf ^Δ/Δneu (KO) brains during postnatal development. (right) A quantification of B-Raf, Raf-1, pMEK, and pERK levels from at least three animals (mean ± SD). Levels are expressed as arbitrary units. Variation among experiments was minimized by normalizing the levels of the proteins of interest to loading controls that showed no change over time (actin). The dividing line in the βIV-tubulin panel indicates that the P18 samples were taken from a different gel. (B) Immunohistochemical analysis of ERK phosphorylation in B-Raf KO and WT brains. The brown staining indicates pERK+ cells. The sections were counterstained with hematoxylin. The vertical bar spans the neuropil-rich molecular layer of the cerebral cortex. G indicates the granular and M the molecular layer of the cerebellar cortex. Arrowheads indicate Purkinje cells.