Activation by point mutation of Ki-ras gene occurring in transfected human normal DNA.

A mutation-activated human Ki-ras gene was detected in NIH 3T3 cells transfected with high molecular weight DNA extracted from peripheral blood leukocytes of a healthy blood donor. A deoxyguanosine at position 35 of the first exon was substituted by deoxythymidine. Nevertheless, cloning and sequencing of seven independent Ki-ras first exons, isolated from the same human genomic DNA used to transfect NIH 3T3 cells, failed to reveal the expected point mutation. Since transfected DNA is susceptible to mutagenesis in mammalian cells, we hypothesize that a base substitution occurred during the transfection assay.