More than 10 different dominant transforming genes (oncogenes) have been identified in human tumours. A human bladder carcinoma oncogene, closely related in sequence to retroviral transforming genes, is split into four exons; the first encodes the N-terminal 37 residues of p21, a protein of unknown function. The oncogene is activated by a single point mutation (guanine to thymine) resulting in the change glycine to valine at position 12 of p21 (refs 3, 4). We report here that the amino acid sequence surrounding this residue is highly homologous to the beta-subunit of mitochondrial and bacterial ATP-synthase in the region of the polypeptide that is believed to contribute to nucleotide binding. Thus, p21 may form part of an enzyme that uses purine nucleotides in catalysis. This is consistent with the finding that an equivalent murine oncogene product binds GTP.