A single genetic alteration, a guanine-to-cytosine transversion, is responsible for the acquisition of malignant properties by K-ras genes of two human tumor cell lines established from carcinomas of the bladder (A1698) and lung (A2182). As a consequence, arginine instead of the normal glycine is incorporated into the K-ras-coded p21 proteins at amino acid position 12. This mutation creates a restriction enzyme polymorphism that can be used to screen human cells for transforming K-ras genes. This approach was used to identify the mutational event responsible for the malignant activation of a K-ras oncogene in a squamous cell lung carcinoma of a 66-year-old man; this point mutation was not present in either the normal bronchial or parenchymal tissue or in the blood lymphocytes. Hence, malignant activation of a ras oncogene appears to be specifically associated with the development of a human neoplasm. Molecular cloning of the mouse c-myb locus and its rearranged counterpart in the ABPL-2 tumor reveals that the alteration in this locus is due to insertion of a defective murine leukemia virus proviral genome containing both long terminal repeats upstream to the v-myb-related sequences. This insertion interrupts the c-myb coding region, which is similar to that observed in the generation of avian myeloblastosis virus.