Carcinogenesis appears to be linked with autogenesis, because, the aberrations in the cancer cell is heritable and have shown to share abnormality in their DNA sequences. The chemical mutagen and X-ray induce aberrations and local changes in DNA that led into cancer.
In normal human beings, in their lifetime, an estimated 1016 cell divisions take place. In a mutagen free environment, a single gene is likely to have undergone mutation on about 1010 mutations per gene per cell division. If a single mutation causes the cell cancer then we would not be viable organisms.
From epidemiological studies it has been observed that the incidence of most types of cancers in human beings increases markedly and exponentially with age.
The laying out of cancer and outburst of cancer are not a simultaneous process. For example, the lung cancer arises usually after heavy smoking of 10-15 years. The incidence of leukemia in Hiroshima and Nagasaki did not show a marked rise until 5 years after the atomic bomb explosion. So, during this long incubation period the prospective cancer cells undergo successive changes.
According to multihit model, cancer arises by a process of clonal selection. The cancers in general seem to arise by a process in which an initial population of the cell or a single cell undergoes mutation and gives it a slight growth advantage. One of the progeny cells would then undergoes a second mutation and that would allow its descendants to give more selective advantage over its neighbors, they become adapted themselves and will continue to divide. Thus a benign tumor may be formed. The third mutation within such tumors would allow it to outgrow the others and progeny would form a mass of cells. All the previous three mutations are remained in these cells and would allow their descendants to escape from their original location to the other tissues through circulation.
Now, many of the specific mutations responsible for the development of cancer have been identified. The concept that the development of a cancer requires mutations in many genes-perhaps ten or more- fits with a large body of information, dating back over many years, concerning the phenomenon of tumor progression. Initial mild disorder of cell in course of time evolves gradually into a full grown cancer.