BRAF is a member of the RAF family that is activated by members of the Ras family upon growth factor-induced stimulation. Active Ras can induce heterodimerization of cRaf and BRAF and this may explain the observed cooperativity of cRaf and BRAF in cells responding to growth factor signals (1). Activating mutations in the BRAF gene are present in a large percentage of human malignant melanomas and in a proportion of colon cancers. The vast majority of these mutations result in a valine to glutamic acid change at residue 599 within the activation segment of BRAF (2).