The humanrasgenes (H-, K-, and N-rus) are members of a superfamily of low-mol-wt GTP-binding proteins that function as G proteins in signal transduction pathways controlling cell proliferation and differentiation (1 ,2 ).Rasgenes acquire oncogenic potential primarily as a result of point, missense mutations in codons 12, 13, or 61, producing single amino-acid substitutions that alter the ability of the protein to bind or hydrolyze GTP. The net consequence of somaticrasmissense mutations is to lock the protein in a GTP-bound, active conformation, thus perturbing cellular physiology and contributing to tumorigenesis. Different tumor types show specificity ofrasoncogene activation. For example, activated H-rasoccurs most often in bladder cancers (3 ,4 ); K-rasmutations are found predominantly in pancreatic (90% cases), colon (40–50% cases) and lung carcinomas (5 –7 ), and N-rasmutations are most often associated with hematopoietic malignancies, particularly myeloid leukemias (8 ,9 ).
