the infrequency of detection of a specific oncogene , more frequent detection of oncogenes in malignant tumors , and failure to observe activated oncogenes in preneoplastic lesions suggest that activation of ras oncogenes may occur as a late and infrequent event in the evolution of some rat hepatocellular neoplasms and that mutation of a specific ras locus is not an obligatory early event in the genesis of these neoplasms .
C0006826
malignant tumors
C0027651
neoplasms