The RAS-RAF-MEK-ERK signaling pathway. (A) The RAS-RAF-MEK-ERK signaling pathway is physiologically triggered by the binding of a surface receptor tyrosine kinase (RTK) to its ligand. This activates RAS and, in turn, RAFs (BRAF and, not shown, CRAF). BRAF-CRAF heterodimers phosphorylate the MEK1 and MEK2 kinases (pMEK), which in turn phosphorylate ERK1 and ERK2 (pERK). Then, active ERKs phosphorylate several substrates in the cytoplasm (not shown) as well as in the nucleus, where they initiate a transcriptional response (eg, through the activator protein 1 transcription complex) that includes cyclin-D1 upregulation and that promotes cell survival and proliferation, as well as feedback inhibitory mechanisms (not shown) to counterregulate pathway activity. The latter, if uncontrolled, can result in neoplastic transformation. Indeed, the BRAF V600E mutation renders BRAF constitutively active independent from upstream regulatory signals and from heterodimerization with CRAF. (B) In vivo activation of the BRAF-MEK-ERK pathway in HCL patients is illustrated by the expression of pERK (red) and cyclin-D1 (nuclear, brown) by BM leukemic hairy cells (counterstained with hematoxylin in the top panel and double stained for the surface B-cell marker CD20 (surface, blue) in the bottom panel). Pictures of immunohistochemical stainings were taken with the 40×/0.85 objective (U Plan Apo) of a BX61 microscope equipped with a DP71 digital camera, using cell^B x.y acquisition software (all from Olympus).