Chemical carcinogenesis is hypothesized to involve manganese superoxide dismutase and gene amplification. Initiation is hypothesized to be caused by destruction of the DNA that enables the cell to induce manganese superoxide dismutase. Tumor promotion then causes amplification of the manganese superoxide dismutase gene and the cell proliferation gene (oncogene) because of selective pressure exerted by the promoter. Because the promoter causes cell division and chromosomal rearrangements, unequal segregation of the amplified genes results. Because cells which have high amounts of the cell proliferation gene and low amounts of the manganese superoxide dismutase gene grow faster, these cells become dominant and a tumor forms.