The multistage theory of carcinogenesis includes the development of multiple activating genetic changes due to exposure to carcinogens, either primarily, or superimposed upon pre-existing mutations in the genome. These changes result in activation of protooncogenes, lack of expression of tumor suppressor genes, or combinations of the above, the sum of which results in malignant transformation. Detailed analyses of chromosomal lesions in bronchogenic lung cancer reveal several recurring abnormalities, including deletions, duplications or polysomy of chromosomes 1, 3, 7 and 20. Aberrations in the short arm of chromosome 3, in particular, are found in many small cell and non-small cell lung cancers, and polysomy 7 is a frequent finding in non-small cell lung cancers. Many of these abnormalities have no identified significance, however the application of current and evolving techniques of molecular biology have revealed specific genomic changes leading to malignant phenotypes in several tumors, for example, the application of polymerase chain reaction amplification techniques has revealed a striking incidence of mutations in the h- and k-ras protooncogenes have been discovered, associated with over-expression of growth factors or receptors, for example epidermal growth factor receptor.