Identifying the Oncogenic Mutation. The point mutation that converts a proto-oncogene into the active oncogene in human bladder tumor DNA was identified by first swapping various segments of the proto-oncogene (normal, non-oncogenic gene) and oncogene using recombinant DNA methods. Recombinant molecules were tested for the ability to induce focus formation in "normal" mouse fibroblasts. Once the site of the oncogenic alteration was limited to a small segment of DNA, DNA sequencing revealed that a glycine codon GGC in the proto-oncogene was mutated to a valine codon GTC in the oncogene at a position corresponding to the 12th amino acid of the encoded protein. In fact, any amino acid except glycine at the 12th position converts the normal protein into an oncogenic protein.