The activated BRAF pathway provides oncogenic signaling to the leukemic hairy cell through the MEK-ERK cascade. Vemurafenib is an inhibitor of BRAF in this pathway. The activity of pERK is reduced as a consequence of BRAF inhibition. The reduced pERK results in decreased cell proliferation. This is measured by immunohistochemical analysis and is used as a pharmacodynamics biomarker of BRAF inhibition. P, indicates a phospho-group; RTK, receptor tyrosine kinase. Professional illustration by Patrick Lane, ScEYEnce Studios.