Altered behaviour or the transformation of a cell can result from the abnormal expression of some oncogene products. Elevated or inappropriate expression can result from (i) mutations in the regulatory region of the gene, (ii) aberrant expression of a transcription factor involved in the regulation of the gene, (iii) gene amplification, or (iv) the insertion of a viral promoter upstream of the gene. In addition, an alteration in the product of a proto-oncogene can lead to the acquisition of a transforming activity. Such changes have been shown to include (i) point mutation, (ii) deletion, and (iii) the formation of fusion genes. Finally, the loss of activity of a gene product can contribute to transformation. This can come about by (i) small or large deletions, (ii) point mutations which abolish function or expression of an intact protein, or (iii) mutations which lead to a protein with an activity which can inhibit the suppressor activity of the normal allele.