RAF1 and cancer: Observations that activating Ras and Raf mutations are typically mutually exclusive2–4, and that only components of the Raf/MEK/ERK pathway rescue growth in “Rasless” mouse embryo fibroblasts (MEFs)5, suggest that the interaction of Ras with Raf, and the activation of MEK1/2 and ERK1/2, may be most critical to Ras-driven cancers.