Ras family proteins, important in the control of cell growth via signaling, are commonly mutated in human cancer. Activating mutations in Ras are a leading cause of resistance to modern targeted therapy, and patients who harbor Ras mutations have considerably poorer prognoses than those with wild type Ras. Targeting Ras has proven difficult because oncogenic mutations activate Ras primarily by ablating enzymatic activity, leaving classical enzyme inhibition strategies unworkable. The high affinity of Ras for GTP---which locks Ras in an active conformation---combined with high intracellular GTP concentrations makes outcompeting the bound nucleotide extremely difficult.