The small, membrane-tethered G-protein Ras plays an important role in many cellular processes, including growth, differentiation, and survival (Wennerberg et al. 2005). Ras acts as a molecular switch which is bound to GDP in its inactive state, and GTP in its active state. When Ras is active, it can directly associate with the serine/threonine kinase Raf, which can be activated by phosphorylation upon recruitment to the membrane. Raf can then activate the dual-specificity protein kinase MEK1/2, which in turn activates the Mitogen-Activated Protein Kinase (MAPK) ERK1/2. Since Ras controls multiple cellular outcomes, its activity is tightly regulated. Inactive, GDP-bound Ras can be activated by interaction with Guanine-nucleotide Exchange Factors (GEFs), which eject GDP from the nucleotide binding site of Ras and allow GTP to bind, which is present at a much higher molar concentration than GDP in the cytoplasm. Examples of...