Amplification is one mechanism for activation of oncogenes and results in an excess of DNA template, which can lead to overproduction of oncogene-specific RNA and protein. Amplification of oncogenes has been observed in different tumor tissues. In certain cases amplification and overexpression of particular oncogenes have been correlated with tumor progression and clinical behavior. The best example is neuroblastoma in which the N-myc oncogene frequently is found to be amplified. Over 1,000 patients with breast cancer have been studied for amplification of the c-erbB-2 oncogene until now. The evidence from the studies that amplification of c-erbB-2 is correlated with poor prognosis is in our opinion not convincing. More and more investigations about oncogenes and disease prognosis will take place rather at the protein level than at the DNA level.