The Raf kinases are important intermediates in signal transduction. Raf protein family members, including A Raf and B Raf, have intrinsic serine/threonine kinase activity. Interaction between Ras proteins and Raf proteins results in Raf-mediated phosphorylation and activation of MEK (also known as MAP kinase kinase). Defects in BRAF are involved in a wide range of cancers. B-Raf is a serine/threonine protein kinase that acts as a signal transducer from membrane-associated receptors to nuclear transcription factors. 1 BRAF is important for the regulation of cell proliferation and determination of cell fate during embryogenesis. BRAF acts downstream of Ras and upstream of MEK in the Ras-Raf-MEK-ERK signal transduction pathway, which is a conserved RAS-activated protein kinase cascade that regulates cell growth, proliferation, and differentiation in response to growth factors, cytokines, and hormones.