The next step is to promote, in this way a cell that has begun its change becomes cancerous. The promotion has no effect on cells that have not undergone the initiation process. Thus, several factors, often a combination of a susceptible cell and a carcinogen, are needed to cause cancer.

Returning to the process by which a normal cell becomes a cancer cell has been that in the end, the DNA also undergoes some changes. These changes in the genetic material of cells are often difficult to detect, but sometimes a change in the size or shape of a particular chromosome indicates a certain type of cancer.

For example, an abnormal chromosome called Philadelphia is found in about 80 percent of patients with chronic myeloid leukemia. Other genetic changes have also been identified in brain tumors and cancers of the colon, breast, lung and bone.

For the development of some cancers, may require several chromosomal changes. Studies of families with polyposis of the colon (an inherited intestinal disorder in which polyps develop and become cancerous) have explained in this way how colon cancer develops: the normal lining of the colon begins to grow actively ( hiperproliferative) because the cells do not have a suppressor gene on chromosome 5 that normally controls the growth of the lining of the intestine.

A slight alteration in the DNA then produces changes that lead to the formation of an adenoma (a benign tumor). Another gene (the oncogene RAS) causes the adenoma grows more actively.

The consequent loss of a suppressor gene on chromosome 18, in the long run, stimulate the adenoma and, finally, the loss of a gene on chromosome 17 benign adenoma turns into cancer.

Other additional changes can make the cancer metastasize (spread). If a cell has already become cancerous, the immune system can often destroy it before it reproduces and is established as a cancer.

Cancer is more likely to develop when the immune system does not operate normally, as in people with AIDS, those taking drugs that inhibit the immune response and in those with certain autoimmune diseases. However, the immune system is not foolproof, because the cancer may escape the protective surveillance of the immune system even when operating normally.

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