ras and human tumors.

Mutational activation of ras oncogenes is frequently encountered in human tumors. For unexplained reasons, K-ras mutations are predominantly found in pancreatic cancer, colorectal cancer and adeno-carcinoma of the lung, N-ras is predominantly found in a subset of acute leukemias and in myelodysplastic syndromes, while H-ras mutations are rare. In most tumors, ras mutations are not clearly associated with specific clinical or biological features, but in lung cancer, childhood lymphoblastic leukemia and possibly in myelodysplastic syndromes ras mutations may predict a poor prognosis. Accumulating evidence suggests that exposure to chemical carcinogens is responsible for many ras mutations in humans.