Activated N-ras in a human rectal carcinoma cell line associated with clonal homozygosity in myb locus-restriction fragment polymorphism.

An N-ras transforming gene was detected in human rectal carcinoma-derived cells (7060) and molecularly cloned. The genetic lesion responsible for the transforming activity of the 7060 oncogene was localized to a single nucleotide transition from A to T in codon 61 of the predicted protein. This lesion in the second exon results in substitution of histidine for glutamine at this position. We also found an EcoRI restriction fragment length polymorphism, consisting of two alleles, of the human c-myb gene. The 7060-transformed epithelial cells showed the homozygous phenotype, while normal fibroblasts of the same patient showed the heterozygous phenotype. This suggests a relationship between the phenotypic change in the c-myb locus and the induction of the 7060 tumor.